My iPhone is unlocked. I went into the Tim store with it.
Although Wind also offers a good deal, Tim seems to have the best coverage for Sicily.
I paid for the SIM card, then took a one-month renewable plan. I added a North American calling option. Then pre-paid 10 euros credit to pay for the calls to N.A. (at a greatly reduced rate). I get 2 or 3G data with it - enough to deal with e-mails and consult Google maps and browse the websites I need (timetables, bookings, attractions, etc.). It cost me 30 euros for the first month and 20 euros for the second month (including a full top-up of the pre-paid credit).
I had to have my passport in hand to buy the SIM card. The gal set it all up for me. I did have to go back the next day to get help fiddling with the phone's settings, until the data plan could get picked up properly. The use of the maps on the go have been priceless!
I wait for when I can use a free wi-fi connection, for uploading photos to FB and to back up to the Cloud.
If you plan on day tripping from Catania to places like Siracusa/Ortigia or Taormina, I can recommend that you consider just taking the train for those days. Service is frequent, cheap, and no parking hassles.