Well I switched to Virgin Mobile and got a Samsung S3.
I just put my Virgin sim card into this phone (iphone 4) and it works fine. I can make a call. It gets a 3G connection and can surf the web and it can send SMS messages just fine as well.
So, I don't know where all this confusion comes from. I paid full price for this phone at the Apple Store a year ago. It's off-contract / unlocked. Just like I said.
If someone wants it, then it's $300 and it looks like you can use it on any network you feel like.
i think much of the confusion stems from the fact that you used the term "off contract" which would imply that the phone was once "on" a contract and is now "off" that contract. your phone is actually an OEM factory unlocked phone
that and people are 3/4 retarded.
FWIW you can absolutely 150% have an iphone 4 factory unlocked once you're done your contract. its a service i believe all 4 carriers are offering by now...rogers/fido were the first IIRC. i have already made plans to do so when my contract ends shortly and have friends who have done it with various carriers. they do some magic on their end, you sync the phone with iTunes and when the process is done the phone is unlocked.