This doesn't appear to be an option in the normal program for editing keyboard shortcuts, but it is stealing one of my favorite emacs bindings. Might be somehow related to Metacity instead of being a general gnome setting. I know this can be fixed because I figured out how to do it at one point, but now I can't remember how. Currently set to Window-m.
As suggested by Nicolas in his comment, I am indeed running Compiz instead of Metacity. He is also correct in that installing compizconfig-settings-manager and disabling the "Negative" plugin shortcut has solved my problem. What a painful way to track down random keybindings installed at the system level...