To make progress on personal projects without wasting the work day, type up notes during lunch break.
For every screenful of text, someone notices the little screen of the PDA and the way it rests atop a portable keyboard. They start talking about it. Admiring the fold-out keyboard. Asking questions. Expressing disappointment that it's not a combo phone & Wi-Fi do-everything wondertoy... This reduces available time, particularly if I don't want them reading what I'm writing and switch to a different app.
So, for the most part, computing during lunch is not effective, but I'll be interested to see how the MacBook compares. More or fewer comments than the PDA?