A particular client's mirror of CentOS exploded today with the
unannounced CentOS 6.3 release. Congratulations to them for such quick
work: is there a nominal release date for SL 6.3, so I can make sure
to allocate disk space?

I've actually got a mock setup, running SL 6.2, chewing its way
through all the upstream vendor's 6.3 SRPM's for my knowledge and
education. (Also, I publish Subversion and BIND updated SRPM tools
over at www.github.com/nkadel/ and want to test them with new
environments.