Like Jdcanb said, there are some ATM's around that will let you withdraw dollars, but you'll get screwed on the exchange rate if you do.
If you want dollars in Tijuana, you are much better off withdrawing pesos from the ATM and then exchanging them for dollars at a local casa de cambio or exchange place. The rates will work out in your favor with both transactions and you'll actually come out ahead a little.
