Special thanks to those of you who tested SP1 RC Refresh 2 (6001.18000). That was our very last release candidate and it was, in fact, the very same bits as the final release. (It was a Release Candidate in the truest sense of the word.) Those of you who installed the SP1 RC Refresh 2 will not need to update to the final version-you already have it.
That should answer your question, Peter
oh, and slip-streamed SP1 DVDs are now available!