When fedpkg srpm makes the SRPM for mockbuild, it doesn't take into account the current branch. On el5 at least, rpmbuild-md5 must be used or mockbuild will fail.
The workaround is to use {{{ fedpkg srpm --md5 }}}
But this should be the default on EL5 and EL4 branches.
Got this sorted out, will default to md5 sum when doing an srpm action from an el4/5 branch.