It was going to be a deadline on January 1st, 2008 that you would need a passport to cross the border, but there's a huge backlog in issuing passports so they realized they couldn't make it mandatory on the first of the year. Now you have to have a government issued photo ID plus some proof of citizenship (like a birth certificate) if you don't have a passport. And they still say they will be changing it to require passports at some point in the future but the date hasn't been announced yet.
I need to renew my passport before this summer anyway because it'll be expiring. And it's still in my maiden name. I should get on that.