Here is a list of packages for 6.3 for which upstream SRPMS are available but which aren’t built on CentOS.
i686 only (these *are* being built, but won’t show up in a x86_64 ls):
iprutils libehca libica librtas libvpd (still available as rpm)
lsvpd powerpc-utils ppc64-diag ppc64-utils servicelog yaboot
python-rhsm (still available as rpm)
rhn-client-tools rhnlib rhnsd subscription-manager subscription-manager-migration-data virt-who yum-rhn-plugin
SRPM available but not built by RH:
kmods that are merged with the kernel:
bnx2fc bnx2i bnx2x cnic hpwdt igb ixgbe mlx4_core mlx4_en mlx4_ib snd_core snd_hda snd-hda tg3
redhat-indexhtml -> CentOS-indexhtml redhat-release-* -> CentOS-release
I’m compiling a list of packages for which the SRPM does not build a package with the same name. If you’re interested in that list let me know.