Surprise, surprise, some people are finding out the hard way that when you switch carriers, you can take your number with you now, but usually not your phone. Besides the incompatibilites in networks (Verizon and Sprint use CDMA, Nextel uses iDEN, and T-Mobile, AT&T Wireless, and Cingular use GSM) most carriers lock their phones so that they can’t be used with anyone else. It is possible to unlock most phones so they can be used on another carrier, thought it’s not always easy, so if being able to take your phone with you is important, you probably should just suck it up and pay extra for a phone that comes unlocked in the first place.
Read [Thanks, Mike]