I just ported my Verizon number over to Google Voice, which also means that I'm cancelling my contract with VZ. Good riddance, what a sucky provider they are.
My plan now is to get a T-Mobile data-only plan (no voice) for $40/mo, and then use the data connection to make calls with my Google Voice number. Should be interesting.
Regarding cost, of course I get hit with an early termination fee by VZ, but because I'm currently shelling out $100/mo to Verizon, I actually end up saving money because the new thing will be so cheap.