There is a direct plug in booster with an external antenna now.
I've been happy with the Zboost, too, but I got a 20% discount before I would buy it. They are still expensive and too easy to steal from your car.
zBoost is sold at
http://www.wi-ex.com/ uses external antenna, a clunky box, and cigarette lighter plug. This is the first generation solution, modeled after indoor house units.
or
Dual Band Booster Solution Kit for iPhone with 24inches magnetic mount antenna. That tall antenna may be a problem for some cars. $249.99
or
http://www.powerfulsignal.com
Wilson 805201 iBooster
http://www.powerfulsignal.com $243 and uses a cradle.
But, whatever you buy and use, PLEASE report your results. I can't find any kind of comparison testing (like Consumer Reports).
Unlike analog radios, the boosters don't make anything louder. Instead, you should see a strong signal all the time (3 or 4 bars), longer battery life (because you phone doesn't have to transmit very far) and fewer dropped calls.
With all the new 3G phones, the cell systems are quickly getting overloaded. They were not prepared for such high demand and most aren't investing in new cell towers. Instead, they are trying to buy out the upstarts and merge, resulting in FEWER towers in operation.
Also, AT&T was getting ready to sell a Cisco cell phone repeater (booster), but stopped for some reason. The web pages got out by mistake and they would not comment further.