e.g. on NetBSD or Solaris? Or does it require exclusively Linux-specific system calls, virtual filesystems, etc.?
Also, if it's currently limited to Linux, what changes would be required to port it to a UNIX OS?
Edit: the reason this is on Superuser is that I was considering it as a way to run a Linux guest system on BSD, for desktop use.
Edit 2: the "user-mode Linux" I refer to is not the GNU userland, but a port of the Linux kernel to Linux (see the user mode Linux wikipedia article for details)
(And from that page it does look like kernel support is required for it to be effective, still not quite sure though?)