Per the instructions at http://prepaidwithdata.wikia.com/wiki/Mexico, I successfully bought a SIM ("chip") from a Telcel vendor in a little technology galeria across from the Torre Latinoamericana for MXN$120 (came with MXN$10 credit) and then recharged it with MXN$200 at the 7-11 in the base of the Torre Latinoamerica. I then texted "alto7" for 7 days/1GB of data.

Trouble is, my hotel's wifi is less than stellar, and so I've been tethering my laptop to my phone (works very well--2mbps and decent ping times) to get work done, but it's running up my data usage more than I expected.

Will there be any issues if I go back to 7-11 and recharge it with, say, 500 pesos and then, once my current 1GB runs out, text "alto30" to get 3 more GB of data to use?

It's a bit more data (and money ) than I was expecting to use, although if I look at it as being the equivalent of ~$7/day for hotel wifi, then it's not as bad (although one of the considerations--other than cost--for the place I'm at now was the fact they advertised free wifi...but with poor coverage in my room and a mediocre connection even if I walk to the end of the hall for a stronger signal, it's still a little annoying).