If you have an expired PASSPORT IT IS STILL GOOD ALL YOU NEED IS A CURRENT DL WITH A CURRENT PICTURE. All passports have specific identification within them that validate who you are, thats why they swiped them. As long as your name and DOB matches up to your DL you will be fine. NOW YOU CANNOT USE AN EXPIRED PASSPORT TO TRAVEL OVERSEAS but for just crossing to Mexico there are no problems. As long as you are nice and not a jerk to Customs they would have no problems with it. I have been using my passport that expired in 2002 for the pass two years and I have not encountered any problems. Lets face it, the border is busy, the sentry is not going to waste his time by giving you a hard time when there is a line of 300 people behind you!