Actually that changes the actual location of the Program Files directory. It isn't just a pointer to a default install path. If you change that, Windows will start looking to the new location when it looks for anything it expects to find in the Program Files. Since you started out with all those files on the C drive, Windows will b0rk because they're not in the new location. The use of the Program Files directory is also a variable in many of the paths pointing to it, rather than a hard path to the C drive and the folder.
Meaning, instead of a Windows file being referenced as c:\Program Files\Common Files\blah.blah it is referenced as %programfiles%\%commonfiles%\blah.blah (common files also being a variable).
Now, you go and change that, and Windows suddenly starts looking for particular files on the G drive and they aren't there. You might get away with copying all the files then making the change and rebooting, but you might also hose the installation.
For instance, the App Paths key in Windows\CurrentVersion. Several of those use the variable. Then, in the Handlers key under Explorer, there are Windows Media Player handlers using the variable.
As far as applications are concerned, some will heed the variable, some won't. Anything that uses the Microsoft Installer is likely to, plus some other very common ones. But you'll still run into apps that will ignore it.
Of course, you COULD create your own new variable, like %originalprograms% and replace all the current references to %programfiles% with that, and make the variable's value point to the C drive Program Files. Then change the ProgramFilesDir setting to the other drive, and any future installations that use that variable will point to the new directory but existing programs or shared files or DLLs will continue to work with the C drive. Of course, simply copying all the Program Files contents to the new location first would probably be easier. But not as cool.
The Powertoys for Windows XP version of TweakUI allows you to change many of the special folder locations, but NOT this one. This is most likely due to the fact that it does point to so many required files, and if you just change it it breaks so much stuff. With the My Documents folder or something like that, you just end up with a new folder where your personal stuff goes and you have to move it from the original location.
