I did try to run user mode linux for centos3. This is my setting:

Host Distribution: rhel3
Host Kernel: 2.4.21-27.ELsmp
Guest Kernel: 2.4.27-1um SMP
CentOS distr: 3 ( http://mirror.trouble-free.net/centos)

I get my centos 3 rootfs by installing distribution in a Fedora Core 3 UML, i.e., I boot up off a pre-built  Fedora Core 3  rootfs, then  mount  centos  file system and install in there. The installation seems OK, though I see a lot of warnings with selinux. And after the installation, I can chroot to new cent os file system.

But when I boot up off the new  centos 3  rootfs,  it hangs at  'Configuring  kernel parameters  [OK]'. Does anyone know why?