Annoying bug of the week: shift+alt+tab being broken while using Gnome Shell in Ubuntu 11.04.
Probably a side effect of upgrading instead of doing a clean install for 11.04, but shift+alt+tab wasn't switching back through the window list in Gnome Shell. alt+tab worked fine still.
Fixed by opening the CompizConfig Setting Manager (alt+f2, ccsm) and disabling the Prev window hot-key under Window Management -> Static Application Switcher.
alt+f2 -> gconf-editor
apps -> metacity -> global_keybinds -> switch_windows = <Alt>Tab
apps -> metacity -> global_keybinds -> switch_windows_backward = *blank* or disabled