Okay... I've been trolling these boards for a while reading everything on the subject and I just have to chime in here with my experiences with Straight Talk.
First of all, it IS unlimited data in all shapes of the word unlimited. Meaning, as far as I can tell, you should indeed be able to consume as much data as you want to use on your phone (not including tethering, which is against the TOS) during the month. HOWEVER, since as many people have accurately pointed out, they have so many "blocks" they have purchased from AT&T regarding data... they use a "excessive usage" tactic to try and scare/discourage their users from using more the 3GB a month. And it IS 3G, not 2GB as has been suggested. When AT&T recently switched their service plans from 2GB to 3GB, this change was reflected in Straight Talk's deal. So.... right now if you go near 3G a month (or over about 300MB in a single day)... an automatic warning is dispatched from Straight Talk who sends both an SMS text message and recorded phone call to your straight talk number informing you that your account may be disabled due to excessive usage and to review tone TOS and call them with any concerns, blah blah blah.
Now as soon as you hit the magic 3G mark data is pretty much instantly disabled from your account, meaning they actually yank giving you an IP number for data whatsoever. It just simply checks with the MVNO server, which sends a "deny service" back to your phone, and neither EDGE or 3G is returned back to the phone. However, they DO NOT (at least in my tests) yank and disable your phone number. When I had my data pulled, my phone and SMS text messaging completely worked and was never affected. I just stopped receiving any "E" or "3G"/"4G" symbol at the top of my menu bar (I have an iPhone 4S) no matter how many times I toggled data on and off or switched back and forth between airplane mode.
Now, don't get me wrong, at this point I was pretty much livid and was scared of the possibility that I would be having to purchase a new $15 SIM card every month to continue using the service (which, amazingly, would STILL be cheaper then using AT&T) but thankfully, after many hours with Straight Talk's amazingly under-staffed and rather stretched thin customer service people... I was given a compromise solution that I hadn't ever heard once on these boards, but is such a workable and surprising solution I'm guessing its how they're currently dealing with new customers such as myself who had very high hopes for the new BYOS option and was incredibly disappointed by all the negative talk I had read on the boards regarding their accounts being deactivated if you went past some mythical unknown limit on was supposedly a truly unlimited service, and quite clearly marketed that way.
After ensuring them that I was not tethering (which I never once did, despite being jailbroken, because I really wanted to play by the rules and test them based on my actual phone data usage in a month) only using apps such as netflix and YouTube to stream content to my phone on long train rides to work every day, a service that is marketed on several of their own straight talk phones which also include unlimited data plans (you cant sell a device with capabilities out of the box and then turn around and say using those capabilities is unreasonable or excessive) and told them that, on average I would use about 4GB a month when I had been with AT&T. I told them that occasionally I would go over and be charged $10, which is why when I heard about straight talk providing the same often great network speeds for less then half the price per month with no contract and no worry of overage fees I jumped at the chance, bought out my contract with AT&T and jumped to Straight Talk. During all my conversations with straight talk I was very patient, I never once yelled (abuse of their reps is clearly outlined in their actual TOS as a non-negotiable fail on your part. Basically, if you're a jerk, your contract is void) and I was very patient in listening calmly to their by-the-book suggestions of me reducing my data usage, and then equally calmly and with great technical detail explaining how I met those requirements but that my normal usage of data in a month would always be somewhere in this range due to how I used my phone on my phone as literally years of usage data on my at&t account could attest to. so then I was told a new term to which my ears perked up to, and that is what they referred to as a "soft bar". Basically, their auto-cutoff system can be programmed with a flexible soft cap. Whenever you approach this soft cap, a warning is sent to my phone. When the soft cap is reached, the data is disconnected. However, if you call them in the interim and explain your usage, why you may have needed more data for your non-tether phone usage that month, they will in turn "raise the soft bar" to a higher threshold for the rest of the month. For instance, from 3GB to 4GB. If you are reaching this new threshold, the same thing applies. You must call them when the auto warning is sent and, after stating your need, the threshold will again be raised another GB and so on. This is actually their rules, and so far, on my second month, they have done exactly that. They have always raised the bar for me, and I haven't been disconnected for data again. Even more remarkable, the data is not throttled whatsoever and is completely comparable to AT&T's extra GB I would routinely he charged for each month. I check my usage using the iPhone's built in data usage calculator in the settings App and each month upon adding another $45 credit I reset the values and start tracking fresh.
This is the way (and likely probably the only legal way) they can advertise unlimited data service through the current deal they have setup with AT&T. If they didn't set a mysterious bar... obviously you'd have the sprint type users who would fully abuse the system and everyone would be left with miserable speeds and crappy service. Their scare method of "excessive use" probably scares enough people who don't truly read the TOS or are actually abusing the service using it for torrenting on their laptop or something as their se Internet connection to conclude that it's not truly unlimited. And granted there are some hoops to jump through to increase your allotment per month. But if you follow the rules and are reasonable in monitoring your data, and calling them to "raise the bar" if you are needing more... they do exactly this thereby giving you truly unlimited data, at full HSPA+ "4G" 3G speeds, with no throttling whatsoever. For $45 a month. Its an exceptional, astonishing value... and while not the no-hassle all-you-can-eat style of sprint, you actually get a service you can use with speeds that average to me around 10MB/s and as long as you care enough to show that you're actually watching how much you consume, you can consume as much as you wish. I think this is a terrific and fair compromise and now that I'm in my third month, felt the need to share what I have learned and my full support of Straight Talk. Their method of "limiting" people may be a tad unethical, but because they absolutely offer truly unlimited data as long as you follow their rules shows to me integrity and truth in their advertising.
Granted, some people may think that calling them every GB of usage is too big of a hassle (and they have a point, it takes about 1HR on the phone with them each time because their reps are stretched so thin), but it's exactly those type of people that Straight Talk doesn't want (and probably can't handle) using their service. For unlimited data with no throttling, it's certainly worth the hassle to me. If you don't want the hassle, you're welcome to pay $160 a month on AT&T like I used to every month. I'm taking the $100+ I save every month and putting it towards a huge vacation. I'll have $1500 extra every year and the pleasure of knowing I'm no longer giving directly to AT&T a company which I despise and loathe.