Fido's coverage partners down there are: Cingular, T-Mobile, AT&T (maybe some others, but those for sure) - so, you at least know these provide GSM service!
You could check these carriers web sites for rate plans and to find out how to get a SIM card to drop into your phone.
Of course, it all depends how much you will use your cell. Fido's US rate is now 20 cents/minute USD, which isn't *too* bad (not as good as the 10 cents/minute of last year, though!). For infrequent use, that might be cheaper than a pre-paid plan. If you're going to be making lots of local calls, though, a pre-paid is almost certainly the right thing to do.
HTH, Jeff