I got the Griffin Power Jolt at the ATT store--works great.
Yep. Got the Jolt too. Relatively compact so easy to fit in to glove box when not needed.
My FM transmitter should arrive tomorrow. I know, I know they ALL suck but its my only reasonable solution for now...
http://www.tuaw.com/2008/05/23/griffin-itrip-autopilot-fm-transmitter-for-iphone/