Because I’ve had to install Ubuntu a lot recently, and I always forget how to do this.
Basically by default Ubuntu binds ALT+LMB to moving windows. This is incredibly frustrating for nearly every 3D application, as this is usually the way to rotate a camera, so most people disable this shortcut.
The simplest way I’ve found is to run this:
gconftool-2 --set /apps/metacity/general/mouse_button_modifier --type string '<super>'
which moves the key to the “Super” key (on a Windows keyboard this is the window key, on mac the CMD key).
And that’s it! if you don’t have gconftool-2 installed (I think it comes with Ubuntu by default, if not just install through apt-get)