Anything that has a data connection can be made into a "phone". Google, voip, SIP and you can start seeing information about that kind of stuff.
Right now, I have an evo with no plan on it, and I just use wifi for calling and google voice for text messaging.
It's hard to explain the whole thing in one sitting, but just research the stuff and it's rpetty easy to do.
If there is a HUGE interest in this, I may add more information somewhere, but for now I think if you're interested it's not too hard to find the info.