[rhelv6-beta-list] Cannot create RHEL6 mock env under RHEL5.

[Howard Johnson]

> Because of the new XZ compression.
>
> It is really a bad news to me.

Your mock host needs an updated version of RPM that includes XZ support.

Fedora Infrastructure uses RHEL 5 Xen domUs for its koji/mock builders. 
As I recall they use the RPMs found at
http://infrastructure.fedoraproject.org/builder-rpms/

Obviously, Red Hat won't support that version of RPM.