Not sure what you are looking for. For the USA, its all fairly simple if you have an unlocked iPhone. I have a carrier unlocked 3G and a factory unlocked 4s. They have been used on both GoPhone and T-Mobile. The detailed instructions and config settings are all over the web. Other than the next paragraph, its no different than moving to another GSM carrier from the country you are in.
Using the 4s as an example. The one and only rough spot is visual voicemail on TMO, to my knowledge its a non-starter as of this date. For MMS, send and receive, go here http://support.t-mobile.com/thread/9292. If you go with a TMO cut mini sim and reception is poor, pull it, make sure its seated in the carrier correctly and remains seated when you push it in. I have run into this issue and I know of several others with the same reception problems. Making sure its seated correctly worked for me.
On the 3G, TMO is Edge only. Good for email, supports Facetime fine, surfing can be painful but I find any phone on 3G data is a wretched experience in the USA.
For travelers covering the East Coast: From south Florida to Maryland, I have found TMO to be a more reliable carrier. I have traveled this route several times both coastal cruising in a boat and by car on 95. I get far better coverage with TMO along 95 and, coming into shore on the boat, TMO always connects further out than ATT. The phones are not the difference as we swap cards frequently and have used many phones over the years.
For visitors seeking good to excellent international call rates, check out SimpleMobile (TMO MNVO). h20 (ATT MNVO) is decent but limited. TMO has a $10/month plan that's decent, ATT is useless. Given that visitors don't have the time and may not have language skills to deal with poor customer service, just stick to TMO.
For frequent international travelers, learn to use Apple's free iPhone Configuration Utility. Its great for saving and loading profiles.