Thanks for posting this info, I had my unlocked iPhone 3GS on tmobile for awhile, I got 200k on the edge network, I switched to h2o wireless ( a mvno reselling AT&T) because their cheaper. I was paying 100 for 1000 mins. Good for 1 yr. but 1000 would never last. With h2o i got a free sim (mins bought thru pinzoo.com) and pay 25/mth prepaid monthly. For 1000 mins cool thing is $ balance rolls over, it doesn't sound like it from the site but they do.
Data is slow 150-200k and burn up the $ quick. Now if tmobile supports my 3G data on my phone that would be very enticing. I currently run around with a Clear iSpot in my pocket. (Another $25/mth unlimited data 6/1 speed unlocked works for all my wifi devices