Just got back from Key West, Grand Caymen, and Cozumel. The AT&T card worked in Key West but not in the other two. I think they've just rigged it so you have to buy the cards there.
Joie-- I doubt that those cards you got will work. They might in the interior of Mexico but probably not where all the tourists go. These places must make a fortune off phone calls and thats why the've fixed it so you can't use a phone card except thiers.
We ended up just sending an email from onboard ship after we left Cozumel. They do have the internet cafe in some places but the one we went to in Key West was just a small dirty cafe with two computer terminals. We chose not to use them since you couln't send pictures.