[CentOS] How to generate UUID for Xen?

The new virtual machine manager in Centos 5.1 displays some weird behavior here and I assume it's because the UUID of one running VM is the same as of a VM that is shut down. I used the same config file template for creating all my test VMs, so several have the same UUID. Didn't produce any problems with xm. But the new VM manager in Gnome shows active and inactive VMs and identifies them by UUID, which results in it trying to mix the status etc. of the active and of the inactive VM. How can I generate a new UUID? I assume there's some checksum in it, so that I can't just make one up?

Kai