Hello,
if you want to run the newest version of mock (0.9.19) with RHEL5/CentOS5, you can use a backported version from:
http://www.jur-linux.org/rpms/el-updates/5/SRPMS/mock-0.9.19-1.el5.src.rpm http://www.jur-linux.org/rpms/el-updates/5/i386/mock-0.9.19-1.el5.noarch.rpm
regards,
Florian La Roche