AT&T with iPhone Pretty Weak
First, AT&T does not offer 3G in Grinnell - the market is probably not big enough for them - so you won't be able to run all those cool apps without the help of WiFi (which I'm guessing is all over campus). Next, despite what the AT&T coverage map says about Grinnell having "good" coverage, we had 3 iPhones on campus for our spring visit and we all had 1 bar most of the time. If I remember correctly, I also had a dropped call, which I've come to expect from AT&T regardless of geography (in stark contrast to the always-solid Verizon service we used to have).
Still, if you really want an iPhone, I'd wait for the iPhone 4 and live with a little bit of bandwidth/coverage frustration in Grinnell. (Afterall, there's no denying the phones are way cool and WiFi will help compensate for some shortcomings.) If you want really solid voice performance, in addition to 3G with less WiFi dependence, I'd go with a DROID-based phone on Verizon.