Enable ALT-TAB in GNOME Classic in Ubuntu 12.10
Turn on the ALT-TAB feature in GNOME Classic on your Ubuntu 12.10 system
Here's how you can enable ALT-TAB in GNOME Classic in Ubuntu 12.10.
To install GNOME Classic:
1. Open a terminal window.
2. Type in the following command then hit Enter.
sudo apt-get install gnome-shell
3. Log out and choose GNOME Classic on the login screen.
To Install and use CCSM:
1. Open a terminal window.
2. Type in the following command then hit Enter.
sudo apt-get install compizconfig-settings-manager compiz-plugins
3. Use this command to start CCSM:
ccsm
4. Click Window Management in the left pane.
5. In the right side, Click the checkbox next to Application Switcher.
That's it. ALT-TAB should now work for you in GNOME Classic.
For more, see the original article at the link below.

How To Enable The ALT-TAB Behavior In GNOME Classic - Ubuntu 12.10/12.04 | Up Ubuntu