We have Verizon and have never had a problem with reception anywhere in Disney World... however one thing to consider is how loud it can be in the parks and at the resorts. We missed several calls just because we couldn't here the phone ringing. If you're planning to separate your party and rely on your cells (like we did) make sure to have it on vibrate and/or make sure it's loud enough to hear over the WDW din!
As long as you are only using your cell phone for dinner reservations or checking voicemail it should work fine. The minute you start talking about the office, work, the plane ride down, all reception will stop, so there's no point in even bringing those topics up....
Register on WDWMAGIC. This sidebar will go away, and you'll see fewer ads.