Re: buying US video content from a non-US store
If you're not near the US, buy your first card on eBay. You'll have to pay a $5 premium or so ($50 card is $55 US).
You'll immediately receive an email with the code - no waiting for the card in the mail.
In iTunes, make sure you're logged out of your Canadian account. Go to the US store, redeem gift card, create a new account using a US address (anything should work).
Then, the account will be loaded. It's prepaid and obviously not tied to a credit card so when your account dwindles, you're out of luck. To top off your account, buy some cards when you find yourself in the US again. Alternatively, you can get them on eBay, though you'll have to pay the premium.
Since the iTunes library can support multiple accounts, this isn't an issue. And since Apple TV doesn't care about accounts - it only cares about libraries - this too isn't an issue.
Hooray!