I've use a Wilson external cell phone antenna on my old "smartphone", (worst cell phone I've ever had the occasion to use by the way but I digress), it definitely helped when using the phone up in the high deserts.

I now use a Blackberry, no external antenna connection on it. After talking to some sales reps from Wilson I learned that the only way to get the benefit of an external antenna with it is to use the booster amplifier along with my original antenna.

Couple of hundred bucks for the booster, haven't purchased one yet.