With verzion or sprint you can call them up and request to have the gsm part of sim slot unlocked for gsm networks. The only downside is that it won't take domestic sims t-mobile or att. It might but from what I've read it will not. It will be unlocked for any international carriers though.
This only works if your on contract with verizon and have paid your bill for 60 days.
Obviously you'll need a prepaid AT&T sim to use the gsm part on it's network if it works that is.