You may Notice in Gnome 2.6 Metacity's new default when you double click the title bar is to Maximize, much like Windows does. If you want to get the shade functionality back. You can change it in the gconf-editor. under /apps/metacity/general/action_doubleclick/titlebar. Change toggle_maximize to toggle_shade.
Sounds simple enough, except gconf-editor on some systems changes the string value when you click on it to a boolean. This is a bug. to fix it, use gconftool.
gconftool-2 --type string --set /apps/metacity/general/action_doubleclick/titlebar "toggle_shade"
I had to restart X before the changes took effect. Hope this helps.