Q: Can I bring my phone with me to a new cellular provider?
A: Probably not. The most important thing to remember is that the different technologies are incompatible with each other.  A phone that works with one technology will not work on another.  Even when a phone may be compatible between two different cell companies, many times the phone is "locked" so it can't be used anywhere else.   Even if you get past the lock, you probably can't use data because it is hard coded to a specific carrier.  It's a pretty safe bet you can't take your phone with you to a new cell provider.

Q: What are the different cellular technologies in the New England area?
A:         (click the acronym for details, thanks to about.com)
    CDMA (Verizon, Sprint PCS, U.S. Cellular) 
    GSM  (T-Mobile, AT&T)
    iDEN (Nextel)
            (the following technologies are being phased out in 2008)
    TDMA (U.S Cellular, AT&T, Unicel)
    AMPS (AKA analog - various)

About.com Cellular FAQ

Questions to be answered as I get time:

Brief History of Cellular in the US?

What about Cellular in the rest of the world?

How do I get started?

Why do you want a cell phone?

How do I choose a cellular service provider?

How do I choose a cell phone?

What's all this I hear about contracts and early termination fees?

Why does it cost so much to upgrade my phone?  I got one free when I signed up.

What are your expectations for cellular service, and then we'll tell you how it really is!

Why can't I use the same cell phone when going from one cell service to another?

Can I use my cell phone in Canada?