Many mice have more than 3 buttons, but there appears to be no method of actually using these buttons in Gnome.  Pretty much every other windows manager (and even Windows) allows a straight forward configuration of these buttons, much like keyboard shortcuts, to perform actions such as raising and lowering windows.

After a lot of Googling, I've had to conclude that there isn't even a method of configuring this through gconf, even though these mice have been common place for well over a decade.