As of February 2011, it seems none of the 3 main operators (Movistar, Claro, Personal) have data plans for their prepaid phone cards. Data plans are only available with subscriptions, which require to be a resident in Argentina.

Update - The above isn't accurate. Data is still available from Movistar, and it's still listed prominently on their website for the Prepago plan:

http://www.movistar.com.ar/planes/prepago/paquetes/

http://www.movistar.com.ar http://www.personal.com.ar and http://www.claro.com.ar sell prepaid SIM cards for about

10-15 Argentinian pesos. These cards can be used for the following 3G data access.

(in Argentinian pesos)

Claro:

1 hour $3 2 hrs $5 4 hrs $7 6 hrs $10

1 day $20 2 days $30 5 days $50 10 days $100

to activate send an SMS to: 454 with: 1h, 2h, etc

Movistar:

5 hrs $9

1 day $10 (upto 1GB)

7 days $59

to activate send an SMS to: 226 with: DIA

Personal:

1 day $10 (up to 1GB)

Special offer: 1 day $1 (first 10MB download at regular speed, next data at 64Kbps)