At the time of writing (Feb 2009) the following actions are required to enable compositing functionality in Metacity, part of the Gnome desktop. This compositing functionality should not be confused with Compiz, Compiz Fusion or various derivitives.
I have to say it works rather nicely on recent hardware although I have found it to significantly slow down on some older integrated laptop chipsets.
1. Execute this (Tells Gnome to use the metacity compositor):
gconftool-2 -s --type bool /apps/metacity/general/compositing_manager true
2. Appended this to the end of xorg.conf (tells Xorg to use compositing):
Section "Extensions" Option "Composite" "enable" EndSection
3. Log out of your session, the changes should have taken place next time you log in.