Simple Mobile was launched late last year in November with a $50 unlimited talk/text/intl text/40 meg data plan.
They have just recently upgraded that to 50 megs but now offer a unlimited talk/text/intl text/150 meg option for $60. They also offer 50 meg additional data packs for $10 and 150 meg addtional packs for $25. This makes them the most data heavy prepaid gsm unlimited nationwide option outside of T-mobile Flexpay unlimited Even More Plus ($70-80 unlimited everything depending on smartphone or not).
Simple Mobile has native T-mobile coverage with no roaming. They do have 3G data which is compatible for Tmobile 3G frequency devices (1700/2100mhz dual 3G frequency. It has an uplink on one channel, downlink on the other. It is NOT compatible with full 2100 mhz up and downlink channel 3Gsm/wcdma devices). Unlocked/unbranded/Att devices can only function in 2G EDGE mode for data on Simple.
Simple Mobile sims work out of the box in T-mobile phones without unlocking and possibly in other Tmobile mvnos too like Tuyo. But it is recommend that they be unlocked for optimum data performance. Whether data can work as well in a locked Tmobile device or Tmobile mvno device as well as an unlocked one has not yet been tested.
There are unsubstantiated rumors that state that Tmobile has bought a stake in the company which may explain possibly the sudden jump in data options. Another similar rumor is that they are now owned by Tmobile. At this point none of the rumors are confirmed.
All other gsm unlimited mvno and other prepaid options are offered by Att. Currently, there are four such options. Att prepaid itself, Air Voice Gsm unlimited, H2O Wireless Unlimited and Red Pocket (launching soon) . Except for Att prepaid itself, none of these options offer data or mms.
As of this writing, the only prepaid national gsm unlimited carriers with data options are Tmobile Flexpay , Att prepaid and Simple Mobile. Only Tmobile Flexpay offers unlimited data.