This is a Gnome Shell feature and it would be nice to be supported in Unity also.
This is documented at http://library.gnome.org/users/gnome-help/stable/shell-keyboard-shortcuts.html.en
QUOTE
Alt+` Switch between windows from the same application, or from the selected application after Alt+Tab.
This shortcut uses ` on US keyboards, where the ` key is above Tab. On all other keyboards, the shortcut is Alt plus whatever key is above Tab.
ENDQUOTE
ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: unity 4.8.0-0ubuntu1
ProcVersionSignature: Ubuntu 3.0.0-8.10-generic 3.0.1
Uname: Linux 3.0.0-8-generic x86_64
Architecture: amd64
CompizPlugins: [core,bailer,detection,composite,opengl,decor,mousepoll,vpswitch,regex,animation,snap,expo,move,compiztoolbox,place,grid,imgpng,gnomecompat,wall,ezoom,workarounds,resize,fade,unitymtgrabhandles,scale,session,unityshell]
Date: Fri Aug 12 16:09:53 2011
InstallationMedia: Ubuntu 10.10 "Maverick Meerkat" - Release amd64 (20101007)
SourcePackage: unity
UpgradeStatus: Upgraded to oneiric on 2011-06-17 (56 days ago)
Having to press Alt+Tab before Alt+ the key above Tab eliminates the benefit of the Alt+` switcher.
Also, the key located above Tab varies from country to country so you shouldn't hardcode that to be `. I don't know if Compiz supports doing keyboard shortcuts like that. I just know that GNOME Shell's alt-tab switcher works pretty well and seems to have similar design goals to Unity's implementation which doesn't work near as well.