UPDATE DECEMBER 2012
The kiosk is no longer there and you have to go the main telcel branch. It is possible to get a micro sim now. Just be patient.
Guy here gives a good account:
Getting Telcel prepaid data plan(internet) in PDC - Nov 2012 - Playa del Carmen Message Board - TripAdvisor
The operative problem is when the only person who speaks decent english isnt there
Rudimentary Spanish is then required and you have to be patient, the staff will wish you just went away ... If you ever wanted to know whan bureaucray was like in the USSR well this is it.
You have to get a ticket to queue, then queue again for the cash teller to pay for the sim, then go back to your rep, then go again to the cash till to pay for the credit, then again to pay for the credit and so on. The guy on tripadvisor claims 20 minutes, but we turned up on a busy afternoon and spent a full hour in the store. There are new tariffs the 1,5 gb for two weeks is now 249 pesos ...