Oneiric provides a utility for this. Forgive the cattiness of this post; I can't for the life of me figure out why Ubuntu continues to ship with these keyboard shortcuts enabled in the first place.

Click on the "Dash Home" icon in the top/left end of your toolbar, and search for "keyboard". Open "Keyboard Layout", select the (typically only) layout in the list (why isn't it preselected for you?), then click "Options" and open up "Miscellaneous compatibility options". Select "Special keys (Ctrl+Alt+) handled in a server". Spend the next thirteen seconds thanking the developer who added this little checkbox, and the next ten wondering (again) why the little checkbox isn't checked by default.

In good humor,
JB