We have an unlocked World phone and intended to buy a US SIM card for our stay and it was a nightmare.
Due to the cheapness of cell phones in the US, almost none of the US cell phone carriers sell SIM cards. We went to four cell phone suppliers in Syracuse on our way down and after an hour and a half of frustration, went to Best Buy and bought a disposable cell phone for $30.00 (including 300 minutes!).
While it is a difficult task to activate it (you have to use another line to activate it...hotel phone...) once it was working it worked well for us to communicate with the other family we were travelling with who already had a US cell phone.
We used NETZero the first time and our international calling never did work. You have to have the company enable international calling and something happened so we could never phone home. But, home could call us, and, you can get a free Canadian phone number with your US cell phone, so your Canadian family can call a Canadian phone number when they are calling you on your US phone so there are no long distance charges.
Most recently we bought two Tracphones (brand name) which worked out just fine.
We have a Rogers phone as well, and despite paying additional money for US minutes you still pay roaming charges which WILL add up. we just turn ours off.
We are considering a phone plan on the two tracphones since I shop in Watertown and Syracuse regularly and we travel to Disney at least twice a year. We can get a family plan for the two phones for $14.99 per month total, no contract. Phones are SO MUCH CHEAPER in the US...
You can buy these dispoable phones at US Best Buy, Walmart, Sam's Clubs, Target, etc. Cheap and cheerful but difficult to activate, you must make the time to do it properly so that all of your calling features are activated.
Sorry to be so long winded...cell phones trials in Disney has been a long process for us!