The current Galaxy Nexus and several other phones handle both AT&T and T-Mobile's 3G frequencies. You are right, however, in that a lot of phones won't.
Those are pretty much your only options on the GSM front. T-Mob has a 30 USD/month plan for 100 minutes and unlimited text/data, but all of their other plans are more expensive than straighttalk for smartphones (probably featurephones as well).
There's an interesting discussion following the post. They're not giving up, apparently.At the end, there were three offers presented, and the other two were bigger than our's. They are keeping more employees than we do, and offer more money. We should get the details of the other offers soon, so we should be able to know more more then. In the meantime, I would like to personally congratulate the winner, Gameforge, and I wish them good luck.