finally got a CentOS 5 domU running under Debian. The xen-tools xen-create-image method didnt work
Trying to bring it back up to date would have been a PITA. It has the RPM versions hard-coded in the script.
However the instructions at http://wiki.kartbuilding.net/index.php/Create_Centos5_DomU_on_Debian_Etch_Dom0 worked a treat.
After following those steps, I converted it from a file-based image, to an LVM, with the following steps:
Manually create logical volumes for the filesystem and swap. I use 40G filesystem LVs and 128M swaps.