[Effendi]

3G in rural Sardinia will be quite hard to be found... Probably TIM has more coverage (both 2G and 3G). Don't go with 3, that's sure. I'd choose TIM or Wind. They both have EDGE (TIM everywhere, Wind sometimes), while Vodafone has not.
If you don't find a shop at Cagliari airport you can find plenty of them in the town.

@ myblogtrainer: Southern Italy is big and the situation changes a lot from place to place. In Puglia Wind has the best coverage, in Sicily I think it's also very good (but not much 3G, I fear). BTW it changes too much from place to place. Also speed can vary a lot...