My thumb seems to stay put most of the time when I play.
Don't know how it's "supposed" to be.
When I play chords, my thumb hooks over the top, which is sometimes called the "baseball bat" grip. I use this when I do vibrato and also when I bend strings.
Check out: Classical picture and Baseball picture
You can read about the two types of grips here.
Hope that helps.
[Edited by Pantallica1 on 07-26-2003 at 12:00 AM]
Sometimes I hit notes only dogs can hear.