When I was in Alicante airport a few days ago, I saw a shop selling various prepaid SIM cards, including some that were branded specifically for the iPad. I think Orange are trying hardest to corner this market, although in my opinion their special iPad tariff isn't even all that great. Their special tariff works out as either 3.5 Euros per day or if you will use it more, then it works out at 35 Euros per month. (There is no minimum commitment). You can pick up a ready-to-go Orange microsim pretty much in any shop that deals in mobile phones.
If you can read Spanish (or want to use an online translator) then see:
iPad: elige tu tarifa / PortalTIC.es
Orange tarifas prepago iPad