I have toyed with this idea extensively and have succesfully executed it over July 4 weekend and Thanksgiving weekend last, netting well in excess of $2000 in vouchers - which will fund most of my leisure travel for this year.

The key is to identify the flights with the most likelihood of overselling. I have a source within the airline who can check the actual overbooking profiles for me, so I have an advantage there. But it is not too hard to figure that the last flight on the Wednesday before Thanksgiving will be packed to the gills!

A lot of this depends on the humor of the staff doing the bumping too. At ATL, I am familiar to the NW and CO staff, so I usually get priority for the bump if I want it. However, some stations (like LAX) can be major pains in the butt about it.