Here's the deal with Yoigo in summer 2010:
Yoigo 3G PAYG prepaid internet in Spain
IData with their MoviData card is trying to get a share of the market by lowering the daily fee to € 1.50.
However
it's not flat-fee. Above 100Mb they charge 6 eurocents / Mb. At 3G speeds it can ruin your prepaid balance in no time.
Cheers!