Is that true? I thought t-mobile sold prepaid sims for use in unlocked phones?
They still throttle you after 5GB of data to dialup speeds. So yes it is unlimited if you are fine with 10KB/s download speeds after 5GB.
Verizon barely lets its prepaid providers offer 300MB of data.
ATT providers claim unlimited but will cut you off after 2GB (cancel your service).
T-mobile resellers do the same as ATT ones, they might throttle, but not sure.
Sprint is unlimited (maybe throttling on Virgin Mobile), but their data speeds aren't always the fastest.
You can bring any phone you want to most prepaid providers if its compatible, but finding unlimited data = no.
If you are fine with under 2GB of data, then I suggest Straight Talk E71 SIM swap trick for $45/mo using the ATT network.
Or just t-mobile's prepaid service for $60/mo for 5GB of data and unlimited talk, text. You could also try t-mobile's $30/mo plan with 100 minutes only if you don't use many minutes but still want GBs of data.
As for $300 phones, used iPhone 4 (not unlocked, but if you go with Straight Talk it can be locked to ATT and still work since they use the ATT network). It has good battery life and camera. Samsung Focus S if you want windows phone 7, most WinPhones have decent battery, don't know about camera.
On T-mobile, the HTC Sensation or maybe the Galaxy S2 if you can find it for $300 (I don't think so). Don't know anything about their battery life or cameras.