For Gnome Users:
System --> Preferences --> Keyboard
Layouts Tab --> Layout Options
Alt/Win Key Behavior --> Left Alt is swapped with Left Win
For KDE 4.x Users:
System Settings --> Regional and Language
Keyboard Layout --> Layout Tab --> Enable Keyboard Layouts
Advanced Tab --> Alt/Win key behavior --> Left Alt is swapped with Left Win
This will swap your ALT key with the WINDOWS key (also called "Super", "Apple" or "Command"). This is useful for a person who Alt+Tab's on a PC but has since switched to macintosh hardware and is not accustomed to the Macintosh keyboard layout.
If you are not using a macintosh keyboard, this *should* have the opposite effect and make a PC keyboard ALT+TAB like a Mac.
I never thought to look in the locale settings for KDE, so this took me quite a while to find. Hope it helps others.
This works on Ubuntu 8.10, Ubuntu 9.04, Kubuntu 9.04. (Intrepid, Jaunty). This should work for both KDE and gnome respectively. Hope this saves some people some time.