It doesn't seem like there are any good prepaid data plans. I hate that there is almost no competition between carriers.
It's not that bad, you have a range of options from GoPhone plans to some other carriers if you're unlocking your phone. I don't talk on the phone a whole lot so it turns out minutes on AT&Ts post-paid plan was VERY expensive for me, even compared to pre-paid's typical 10c/minute.
Simple Mobile is a T-Mobile MVNO, so you'll get 3G only for iPhone voice calls. EDGE for iPhone data. h2o wireless is an AT&T MVNO where you will get 3G mobile data. $40/mo for unlimited calling on either of these carriers, compelling if you're constantly on the phone. (The bundled data @ $50 is very minimal.)
Since AT&T is buying T-Mo, your statement about lack of competition may be a self-fulfilling prophesy. If it wasn't for this announcement, I would believe there's just enough competition out there for the smaller carriers/MVNOs to drive smartphone plan prices downward over the next year or two.