I almost hate to admit it now, but I generally do not. I am of course always polite and courteous, but unless something special is provided by the driver I don't tend to. I have not had any hotel shuttle drivers in recent memory that I have tipped.

Funny, I do tip cab drivers anywhere depending on their courtesy, driving, etc. Maybe I need to rethink the shuttle driver tips now that I'm thinking about it. When I travel with my wife she always tends to tip, she is much more generous than I am.