What's fun is when you get to the Canada/US tax treaty and they use the term "spouse" within it - whose definition, the Canadian one or the US one? If that isn't litigation fodder, I don't know what is.
Oddly enough, the US Supreme Court is hearing a Defense of Marriage Act (DOMA) challenge under tax law (an extra ~$350k tax bill because they wouldn't recognize the same-sex marriage). It's even MORE complicated if you live in California - the IRS lost the case there, as California has "community property" rules and under long-standing case law, community property has always granted income splitting for any couple, including unmarried heterosexual couples. So even though the IRS does not recognize same-sex relationships, they DO recognize community property rules for same-sex relationships in those states with such rules. The same will now apply in Washington State as well, where same sex marriage was able to win at the ballot box. Community property rules have traditionally been a western US practice, while they are unusual in the eastern US.
But yes, you were told right - you file as single in the US - or Head of Household if you have children, even though you file married in Canada.