They start putting the decorations up in the begining of November, and they try to get all of them done by the day after Thanksgiving.
And MagiKitty, of course you can visit the resorts to see the decorations! We don't go around the holidays, but one of our favorite things to do on our summer trips is visit all of the different Disney resorts. They're all so unique and beautiful. You can visit the three monorail resorts easily by parking at one and then taking the resort monorail loop around to the others, but it's easiest visiting the other resorts by car. I don't think there is direct Disney transportation from resort to resort, so after each visit to a resort you'd have to take a bus back to a park or the TTC and then take another bus to a different resort. It's definitely a lot easier to drive, and parking is free! Have fun!