Sounds like a good idea. I would make it very short and sweet though as most principals I have talked to "joke" about having too much to read anyway and they can just add stuff like that to their pile.
Still, it would set you apart and he'd remember your name.
I just finished student teaching and did a thank you letter too. Just about how I appreciate all of the support we were given, etc. (And added that I would LOVE to be a part of the team at that school!)
edited to add that my supervising teacher also suggested I send a short email if he had a seminar or meeting with us stating how much I enjoyed it and pointing out something that I learned. (She said she could write a book about sucking up! HA!)