Free for private and educational use, $12 USD otherwise
USBDLM is a Windows service that gives control over Window's drive letter assignment for USB drives. Running as service makes it independent of the logged on user's privileges, so there is no need to give the users the privilege to change drive letters.
It automatically solves conflicts between USB drives and network or subst drives of the currently logged on user.
Furthermore you can define new default letters for USB drives and much more.
Get the installation and configuration instructions here: http://www.uwe-sieber.de/usbdlm_e.html
Disclaimer: As with everything else at Cool Solutions, this content is definitely not supported by Novell (so don't even think of calling Support if you try something and it blows up).
It was contributed by a community member and is published "as is." It seems to have worked for at least one person, and might work for you. But please be sure to test, test, test before you do anything drastic with it.