Pieter pointed us to rpmstrap. It took me 5 minutes work to get an empty Xen FC4 instance with it.

Took ME 5 minutes , got no clue how long my computer has been working on building the actual image since I started it yesterday afternoon and didn't look back at it till late in the evening.
After deboostrap we now have tool that can build Fedora Core 2, Fedora Core 3, Fedora Core 4, Yellowdog 4, CentOS 3, CentOS 4, Mandriva and Scientific Linux systems. Whow that's about all of the most important rpm based distros except for the Suse versions.