I switched from being a pretty long-term Verizon customer to AT&T because of the iPhone. In my area, which is pretty mountainous, I have noticed very little difference in call coverage or dropped calls. There are areas where Verizon had better coverage but there are now areas where AT&T has better coverage. I have seen reports that AT&T bought from Verizon a goodly number of cell towers in rurual areas. I am hoping that increases remote area coverage in the next year or so.
The one thing I did notice was that, when I was in my office, if I held my Verizon Treo up to my left ear, the call would drop 9 times out of 10. With my iPhone, I can listen to calls out of both ears.
I guess my ears were made for the iPhone. I'm still trying to find out what my mouth was made for . . . .