Here are the culinary highlights from our Tuscany trip in April 2003.
1st is another vote for La Chiusa. You'll be forever sorry if you didn't try it out.
http://www.ristorantelachiusa.it/
You can also stay there overnight and enjoy the dinner even more.

2. Badia a Coltibuono: http://www.coltibuono.com/0ristorant...g=prenotazioni
Marvelous scenery, excellent restaurant and great wine! The countryside can't get much better than that.

3. In Siena, La Buca di Porsenna
http://www.florence.ala.it/la_buca_di_porsenna/
Excellent service, delightful ambience, and a very good introduction to Tuscan cuisine.

By far the best meal I experienced in my recent trip to Italy was at Osteria Nonna Gina in Siena. I especially recommend the gnocchi and pici.

For a view and good food it is hard to beat Relais Vignale in Radda in Chianti, just be sure they are serving on the outside terrace of the hotel.

One of the other posters mentioned Volpia, which I would heartily second.