Hi,
The big mess today in the 5.5/updates tree was caused by a cache corruption in the master CentOS tree during the metadata creation.
<tech> createrepo is run with the --cachedir flag. Something went bad :( </tech>
The wrong cached metadata for the last updates (see centos-announce mailing list) caused the issue you have all noticed today. We are sorry for that.
I initially though that it was a out of sync issue, but I was wrong.
The cachedir was cleared, new metadata generated and pushed to the main centos.org server and going down to the public mirrors.
All should be back to normal soon.
Tru