It is possible to unlock it, but since the ESN is what is used to register a phone on a CDMA network and it is hardcoded into the phone, you'd have to get the provider to accept it.
It is much more fun, as a cell phone aficionado (read, geek), to use a GSM provider. The phones are easier to mod and to upload to; many CDMA providers are starting to lock out home made ringtones, wallpapers, etc., as they want you to pay for them through their service. It is nothing to go from Cingular to SunCom to T-Mobile, etc., with the same phone. Try going from Sprint to Verizon to Alltel...
BB