Mexico SIM cards
The AT&T SIM for the USA will also work in Canada and Mexico as long as you first visit (and use your SIM card in) the USA. Your full call and data allowance can be used when travelling in Mexico and this will usually be an easier option than buying a separate SIM card locally after arriving in Mexico. However, AT&T SIM cards are intended primarily as SIM cards for US residents and extensive use outside the USA may violate your terms of service.
Although Mexican mobile service is relatively good value, the pricing plans for Mexican SIM cards can be more confusing than in many other countries as call charges are based on the local area where your sim card was activated and you many be charged roaming rates even for using your phone in another part of Mexico. You can avoid these issues simply by visiting the USA first and using the AT&T USA SIM card while visiting Mexico but for a longer stay in Mexico (for instance if you will be working there) you will need to buy a Mexican SIM card locally upon arrival.