The KB article for SP1 (KB2817429) has been updated with the following:
We have recently uncovered an issue with this Service Pack 1 package that may prevent customers who have Service Pack 1 from deploying future public or cumulative updates. As a precautionary measure, we have deactivated the download page until a new package is published.
SharePoint Server 2013 Service Pack 1 appears to have been pulled from the Microsoft Download Center.
The timing is slightly off, as clients have been requesting this patch lately as its been available for a bit now. I’ve had a few quibbles with Service Pack 1, particularly the poor implementation of registry permissions and general provisioning problems with Service Applications. Its nothing that can’t be overcome, but hopefully the Product Group can clean it up when the patch reappears.