After the recent upgrade to 4.10.2 I noticed that hitting Alt would open 'a version of the task switcher' (which later turns out to be the new one to flip through app windows). This meant that ctrl-alt-arrow didn't work, among other things.
I couldn't reproduce it with the guest session though, until I touched the shortcut, and then reverted it back to default. At that point Alt opened up the switcher just as with my regular session.
This was discussed on #ubuntu-desktop, and confirmed by a couple of users on !us layout. So, defaulting to '<Alt>grave' doesn't seem to work too well, unless some method to work around the issue of different layouts is found. For instance on 'fi' layout the key above Tab is 'section', and something else on others.
ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: unity 4.10.2-0ubuntu1
ProcVersionSignature: Ubuntu 3.0.0-9.14-generic 3.0.3
Uname: Linux 3.0.0-9-generic x86_64
Architecture: amd64
CompizPlugins: [core,bailer,detection,composite,opengl,decor,vpswitch,compiztoolbox,imgpng,snap,unitymtgrabhandles,mousepoll,place,resize,regex,wall,move,grid,gnomecompat,session,resizeinfo,animation,fade,expo,scale,workarounds,ezoom,unityshell]
Date: Fri Aug 26 15:41:17 2011
InstallationMedia: Ubuntu 11.04 "Natty Narwhal" - Release amd64 (20110427.1)
ProcEnviron:
PATH=(custom, user)
LANG=fi_FI.UTF-8
SHELL=/bin/zsh
SourcePackage: unity
UpgradeStatus: Upgraded to oneiric on 2011-07-05 (52 days ago)
I have this issue with a french canadian keyboard layout. There is no "grave" key on the keyboard (only a dead grave key).