If you're driving over the border, you don't need one until 1/08
http://travel.state.gov/travel/cbpmc/cbpmc_2223.html
I just came back from Mexico, showed my DL, and back to US soil I went

I wish I'd brought something good back though. They asked what was in my suitcase: "Personal items". "OK, have a good day". I'd be a great mule.