You're Spending Too Much Time on a Tablet PC When...
...you're reading a book and you catch yourself glancing at the bottom right corner of the page in an effort to tell what time it is.
Heh! I just read that quote from David Gaw over at Cuz We Said So. It gave me a good chuckle because I've caught myself doing that, AND checking to see if I had a 'new mail message' icon!