[CentOS] UML under Centos5

Has anyone been able to get UserModeLinux to work under CentOS5?   I've
got an image which works fine under CentOS4.4 but when I try it under
CentOS5 then it hangs just after mounting root

   kjournald starting.  Commit interval 5 seconds
   EXT3-fs: mounted filesystem with ordered data mode.
   VFS: Mounted root (ext3 filesystem) readonly.

Now, I'd read this was a known issue because of a UML bug which newer
host kernels exercised, so I downloaded the 2.6.21-rc7 kernel and tried
using that as a guest, but still no luck.

Host kernel:
  2.6.18-8.1.1.el5

Guest kernel:
  Linux version 2.6.21-rc7

This is on an SMP box, if that makes a difference, but it shouldn't because
CentOS 4.4 ran just fine as the host.

I'm running out of ideas :-(

-- 

rgds
Stephen