The important thing is not to surrender your I-94 form when you leave the US for your excursion to Mexico. Remember the I-94 is normally pulled by the airline when you check-in for the flight. You may therefore have to explain to the check-in clerk what you're doing. Alternatively keep the I-94 separate from the passport when you check-in so they won't even be tempted to take it.
You will need to show the I-94 with the passport when you re-enter the US. I've never done it with Mexico, but have done this with Canada on several occasions. No real problem.