I have no intent of hooking up an external antenna. I carry my phone in my pocket, and I've never had a problem with the signal.

I suppose you could get interference with a GSM phone, since they run on a frequency that screws with the radio. If you have a GSM phone, i'm sure you've heard that weird beeping noise.
My phone is CDMA, so i dont have that problem.

Also, I'd have to give up my cupholders, which isn't going to happen.