I was calling it the 'window movement key' (based on the Window Preferences settings from Ubuntu 11.04). It seems in practice it's actually referred to as the 'mouse button modifier'.

Install the 'Configuration Editor' (gconf-editor) from the Ubuntu Software Center. You can use it to change the key.. or manually do it from the terminal with:

gconftool --set --type string /apps/metacity/general/mouse_button_modifier "<Super>"

The default value is <Alt>. I've changed it above to be the super (Windows) key. Other valid values are <Shift>, and <Control>.

Here's the description of the key from gnome.org:

"Clicking a window while holding down this modifier key will move the window (left click), resize the window (middle click), or show the window menu (right click). The middle and right click operations may be swapped using the "resize_with_right_button" key. Modifier is expressed as "<Alt>" or "<Super>" for example."