Not sure on the details, but:
Make the partition, either on another hard disk, or on your existing drive (which may require a reinstall). Call it 'Users'. The name doesn't really matter, its just to make things less confusing.
Get into the terminal, and make a symlink from /Volumes/Users to /Users
Then get to know chmod... you will have to get the privliges all straightened out. Don't forget, you gota remove or rename the origional /Users directory first.
This isn't a great way to do this, BTW. I'd recomend figuring out where Mac OS X stores hard disk mounting tables (in linux, its /etc/fstab, but this isn't the case in Mac OS X), and then just add something like:
/dev/sd0w /Users hfs rw
to it. Again, not at all sure where this file is or if its similar in structure.
You could do this all manually with the mount command, but this has to happen before you login on boot up.
My low confidense remarks didn't help, did they? LOL. sorry about that. But someone here will know how to do it!