bargsbeer is incorrect on this one. As stated in the product introduction keynote, the iPad is sold UNLOCKED and will work with any carrier that supports their 3G frequencies (see tech specs page for more about this) **AND** (this is the key part) the micro-SIM format the iPad uses.
The SIM is "locked" I guess you could say to AT&T, but (in theory) you would bring it to Canada, get Rogers (perhaps) to sell you a compatible Micro-SIM, then pop it in the iPad and it would work.
The problem with your plan is threefold:
1. None of the carriers have yet announced support for the iPad's Micro-SIM. Regular SIM cards (even AT&T ones) won't work.
2. None of the carriers have adopted the "PAYGO" data model AT&T has, and no your AT&T data plan won't work (unless you're REALLY REALLY close to the US border and can pick up US cell signal).
3. No announcement from any of the carriers about which of them, if any, will support the iPad (though Rogers is a pretty safe bet), when they'll have Micro-SIMs, what their rules for support will be, and most importantly PRICING.
If I were you I'd wait a bit.