[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 :-(