Scott McLeod at Dangerourously Irrelevent posts something dear to my heart - the act of walking out on presentations that are less than engaging.
We have all been there... the presenter is reading a PowerPoint slide deck to us, they're unprepared, they trip over their words, use lot's of 'ums and ahs', etc. Frankly, it drive me nuts. I always give the presenter the respect they deserve, however, I will flee (with my hair on fire) any presentation that is not useful, engaging, or enthralling.
If I don't depart in a rapid manner, I will turn to other work on my laptop and engage in something a bit productive. Sometimes I will even make notes about what I shouldn't do in my presentations based on my observations.
So, if I walk out, don't take it personally. You are just not meeting my needs, but you are probably doing so for others.
EDIT: Doug Johnson at The Blue Skunk Blog has some additional points on the topic as well.