What's the best pre-paid SIM card to use while in Cancun, Mexico?

I'm going to Cancun and I have a unlocked iPhone 4g. I'll be there for a week and probably only need 100 minutes. Also, I'll need a very small amount of data (1MB?) to turn on data roaming only to send a few data packets. I'm looking for a cheap outbound and 90% will be inbound calling. I've seen some services with free incoming for other regions. Can anyone suggest a good service to use - I believe the main cell provider is Tercel there. Any input would be helpful! Preferably I'd like to buy the SIM card ahead of time in the US and shipped. Thanks!