Microsoft published this message:
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.
Tobias Zimmergren has some more infos on that topic written down.
An Update will come sooner or later. So it might be better to wait for the update before installing it.