You can't use an iTunes gift certificate to buy applications on the Canadian App Store. I've experienced this myself, but never really looked into it before now. But it's true. There's even a thread about it on Apple's Discussion Board.
Highlights include the difference in iTunes App Store Terms of Service between the US:
"10. PAYMENT METHODS. The Service accepts credit cards, payment through your PayPal account, and iTunes Cards, iTunes Store Gift Certificates, and Allowance Account balances as forms of payment."
"10. PAYMENT METHODS. The Service accepts credit cards as the form of payment."
Further down, we get what's purported to be a response from Apple on the issue, which includes:
Due to tax laws and commerce restrictions for software in Canada, customers residing in Canada may only purchase games and applications using a credit card.
So, not only do we get hosed on data rates, don't have access to US TV networks on iTunes (nor Hulu, etc.), not only do we have to pay more to "upgrade" music to iTunes Plus, but if by some miracle some kindly soul gives us an iTunes Gift Certificate, we can't even use it on apps?
UPDATE: As of June, 2010, iTunes Gift Cards can now be used to buy apps in iTunes Store Canada.