May I suggest the Droid Incredible?
I don’t think Sprint has a limit right now on their data plans. I think they do on their broadband data plans (like for PC cards), but not on cell phone data plans. However, Sprint is going to charge $10 for the 4G/Premium content which will also be unlimited and give you priority in data pools (this charge will be added to at least all new EVO owners from what I understand). Sprint right now is going to be charging $30/month for tethering with the new EVO (last I knew) which I am getting tomorrow at 8AM. However, I think this will go down in price eventually.
Droid is just the only option if I leave AT&T. I’m very impressed with every single thing it does, I just can’t get over the “rough” touch acceptance/scrolling.