I'm constantly switching phones. The exclusion is my Pearl which I received from my work which is on Verizon, but my Storm is unlocked and running on AT&T, so I just switch the SIM card between the iPhone and Bold/Curve/Storm whenever the mood strikes.
When my office was with cingular/at&t i would switch a lot because I would test my account on different phones and learn as much as i could about all our models. Now we are with version they won't give me or any of IT access to our account or put us on the account to switch phones and move phones between lines so I don't do that anymore.