Dragonman,

I have never been to one of those fancy hotel spas, but for other hotel services (like meals and room service), I generally do not tip if 10% service is already included. Granted, a massage is a bit more personal than being served a meal. I might give the therapist a couple hundred baht if he/she was nice.

As for traditional massage places where there is no service charge (i.e., the 150 baht/hour places that are all over town), I usually would give a 100 baht tip to the therapist for a two-hour massage.