We tried that with Vista a while back - never could get it working correctly, and I clamored for it in vLite for a while three, then talked to a guy from MSFN forums who was gonna work on it on his own and after a while it just never panned out.
I am not sure why it was so difficult - with XP I nLite'd the OS so that by default the installation created the Documents and Settings tree and the Program Files tree on a separate physical HD during install - and I am pretty positive this is the exact kind of thing you're looking for.
I, too, would love to be able to do that - however, in retrospect, I actually like it without having that.
I install normal apps to the System drive, but I manually install games to a separate physical drive - thereby bypassing the permissions factor on that separate drive, since the Program Files folder is created manually by me and thus is not subject to the strict permissions limitations imposed by both Vista and now 7.
however, it would be nice if I could 7lite the OS so that I could by default have my special folders (aka Documents, etc) already located on the secondary HD, as I manually do it now - all those special folders that are normally under the users\{username}\ tree I manually move on every install in both Vista and now in all my 7 installs to my separate physical HD, making a reinstall of the OS and apps a lot easier for me....
I can try to get back in touch with the guy from MSFN and see if he ever figured it out - but hat might be a good place for you to go looking as well....
