My $0.02:
Milano - A city that's more for residents than for tourists. Though there are plenty of business tourists. The centro is cool. And if you're into fashion and have the cash, it's a great place. But otherwise it's good for people-watching. Food is so-so at best by Italian standards. And get advance tickets to see the Last Supper if you plan on going.
Roma - You'll trip over ruins and monuments everywhere. Some great food and a few outstanding coffee places. Everybody sees the Vatican, the Roman Forum/Coliseum, etc., and you should. But there's so much to cover. It's a great place to get into (church) architecture -- like Bernini and Borromini -- and sculpture. I really like Piazza Navona, the Pantheon. But there are a lot of side places to go and things to see that are off the main drags.
Firenze - I was there last May, and it was a little bit of a let-down after the last time I was there ... which was almost 10 years ago. It was horribly overrun and choked with tourists. Scaffolding everywhere -- more than the usual Italian standard. But it's a great place to go if you want to see all things Medici. Read up on your Renaissance history first, and you'll come to appreciate the Botticellis you see that much more (and if you go to the Uffizi, call ahead for reservations -- it will save you a couple hours in line). And some of the best cheap food in all of Italy. Siena is definitely worth at least a day trip -- I stayed there last time and enjoyed it much more than Firenze (and it wasn't because I caught the last Siena-Juve game of last season there

).
Venezia - Generally expensive, but out of this world. The tourists cling to the Ponte Rialto and Piazza San Marco, and most of the residential units are owned by foreigners -- so expect a huge tourist/outside population. That said, one of the best places to get lost anywhere in town. Avoid the expensive gondolier cheese and take the traghetto across the canal instead for just 0.40 EUR -- mostly the same experience but without the robbery. You'll spend a bit on your hotel, but you can get a lot out of just walking around and eating in places where the tourists aren't -- and there are plenty of such places if you look for them.
Torino - My first time there in May was an awesome trip. The area by the delle Alpi is pretty desolate and depressing, and the air pollution in town is pretty bad ... what you'd expect from an industrial city located in a valley below mountain ranges. Good walking around (a lot is being upgraded for the 2006 Olympics) and plenty of really cool pedestrian-only areas. The downtown boulevards and Baroque architecture is beautiful -- and you feel far removed from tourists. Of course, there's not many tourist things to see -- if you're into that. But it's a great city to just hang out and absorb for a while. Prices are also reasonable -- as everyone has to live there! And go to the Juvestore on the pedestrian-only Via Garibaldi downtown, of course!