I've gotten word that it's actually easier and faster to take a bus from Florence to Sienna instead of the train. Big advantage: the bus drops you off in the center of Siena, not on the perimeter. Feedback please, I'm going in October.
Shelley,
It is all true. Siena has better bus conections than train conections. See http://www.mega.it/ita/not/sita.htm for bus between Florence and Siena.
Yup, the bus is much better. You want the SITA express bus from Florence. One other option is the Grayline tour. It leaves from a stop about 300 feet from the main Florence train station and goes first to San Gimignano for about an hour visit and then it goes on to Siena for about 4 hours. At Siena, you will go on a tour around the town and through the main church. 51 Euro per person. My wife and I took this option and it was quite nice. If you are interested in this option, I can provide further info.
There is also a train that takes about an hour, and then a local bus that will take you directly into the old city. It's easy if the bus timing doesn't work out.
no question, bus over train from Florence. I did it and recom it.