The bus between Florence and Siena is much easier that the train and also you will be dropped in the centre of Siena, instead of outside the city walls where the train station is. www.sitabus.it has train timetables. Once you get back to Florence you can also get a SITA bus straight out to the airport (20 mins)
Sorry I meant www.sitabus.it has BUS timetables!
Thanks for the info. I think you're right a bus from Siena to Florence is a good idea.