Roam Mobility aims to help Canadians save a lot of money on voice and data roaming fees when they visit the U.S. Here's how it works -- you take your unlocked GSM iPhone with you to the U.S., and instead of paying between $1 and $30/mb roaming fees from the big carriers, you simply pick up Roam Mobility microSIM card, slap it in, and start saving. (You can also get data sticks, dumb phones, and miniSIMs as well.)
There are several plans and pricing options, from pay-as-you-go to pre-paid, and you can get both voice and data services. You don't have to sign up for a contract. Check out the link below for all the details.