Just curious, but why the need for unlocked? do you plan to use an AT&T phone on T-Mobile's service or vice versa? If not, then you really don't need the unlocked. If you know you want to use the AT&T service, just buy an AT&T locked phone. Only other reason is for international use.
As far as cheap android phones, there are a number of threads here that have lists of phones at various prices (of course, those prices always are in flux). Here's a good place to start: http://www.howardforums.com/showthre...aight-Talk-sim
I got my phone (Xperia Neo) from Amazon. My own analysis of the reviews from others revealed...
That most sellers get locked phones, unlock them, and resell them as unlocked. This leads to the most common complaint; that the box was opened or that the phone is really used. Also, this is why some phones are sold without warranty; because of the way in which they were acquired, unlocked, then resold.
If you don't pay attention, you can accidentally buy the "International," instead of the "American" version of a phone; then wonder why 3G doesn't work.
I specifically chose the Xperia Neo, because it was only sold in the US direct from Sony, so I knew it came unlocked, directly from the manufacturer.
And, I wanted an unlocked phone because I wanted to minimize carrier specific bloatware, plus it gives me the freedom to move between AT&T and T-Mobile at will, by simply changing the SIM.
If you are really paranoid, only buy items marked "Sold by Amazon."