What are the options for pre-paid SIM cards in Uruguay (Montevideo)? What are the options for pre-paid data plans/packages?
Data via cellphone is one of the most competitive marketplaces in Uruguay.
Vendors are shifting from 3G to 4G, starting in the area around Montevideo. From a consumer's standpoint the only discouraging trend in this market is the adoption of data volume caps by all vendors. As of August 2012 no vendor web-site offered an unlimited mobile internet data plan (the closest was an "unlimited during nights and weekends" from Claro.)
In terms of prepaid data plans, there are several available, from Claro, Antel or Movistar. I won't copy and paste verbatim as I'm sure they're likely to change with time, but an up-to-date wiki is maintained at Uruguay Prepaid Plans on APNCharger.org.