Jan 2008 is still slated for mandatory possession of passport or PASS card, or maritime ID, to cross back into the US by car or walking, so plan NOW for your passport and/or Pass card.
Note: The pass card is credit card size, and is used only for the border areas of Mexico and Canada. It is not used for traveling into the interrior of Mexico, or further North into Canada.
Of course, if there is another terrorist incident here in the US, you can bet that the borders (North and South) will snap shut faster than a virgins hip-hugger pants on prom night, in a hurry due to the populace demanding that they be protected.