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:
10-15 Argentinian pesos. These cards can be used for the following 3G data access.
(in Argentinian pesos)
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
5 hrs $9
1 day $10 (upto 1GB)
7 days $59
to activate send an SMS to: 226 with: DIA
1 day $10 (up to 1GB)
Special offer: 1 day $1 (first 10MB download at regular speed, next data at 64Kbps)