As MikeB said, we got married at the Chateau du Chatelard in Montreux recently, which was fantastic. It's a relatively small venue, we had just over 70 guests for the ceremony and meal with more guests in the evening. Accommodation is also provided. There was great flexibility in what they offered and what we wanted which allowed us to pick our own cake, band and photographer etc.
I know, as someone has already mentioned, that Chateau de Chillon is also available, but there are more restrictions imposed with getting married here.
Hotel Mirador (
http://www.mirador.ch ) on Mont Pelerin (above vevey) has just been refurbished and packages start at about 200 chf per person.
Wherever you do choose, be careful about bringing your own wines to places that already provide it... there is usually a corkage fee to pay.
Also, I think you will be surprised at how popular winter is to get married. Even though we got married at the end of Sept, by March earlier this year, when we were deciding dates, there was only one available Saturday in the space of about three months!
Good luck and pm me if you need more info.
Tola