Microsoft has taken down the ability to download the newest service pack 1 release for SharePoint 2013 with the following statement:
Notice 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.
From Todd Klind'ts SharePoint Admin Blog: "Bill Baer has stated that the MSDN ISO that has SP1 included is not impacted by this issue."
So if you have the ISO downloaded that has SP1 included, you can continue to use that as an install source. We would recommend that if you have already downloaded SP1 that you do not install it yet until this issue has been resolved.
The corresponding KB article can be located here.