The RH6.1 installer ignores errors mounting filesystems with {usr,grp}quota option. On my case, /usr had usrquota and grpquota. Upgrade went on without errors untill it stoped with not enough disk space on root partition. Aparently the kernel lacks quota suport. This seems severe because if / actually has enough space and upgrade goes on a totally inconsistent system is generated.
This issue has been assigned to a developer for further action.
I would recommend that you try the updated images that we just released, as there is a fix there that should solve your problems. The installer was not mounting partitions which had anything other than "defaults" in the fstab options field, therefore partitions with the quota flags were not getting mounted for the upgrade. Reopen this bug is this does not solve your problem. Check out the errata page for the latest information about the new boot images (support.redhat.com/errata)