So yesterday Microsoft added a notice below to the SharePoint 2013 SP1 KB article 2817429
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.
The SP1 download package is no longer available and the official advice is to NOT install the SP1 update package in your production environment until further notice.
However the SharePoint 2013 SP1 installation image from MSDN is apparently not affected so you can still use this image to create new SharePoint 2013 installations (as with my development environment video series).
The reason the update was pulled is that there might be issues with this particular package that could prevent further cumulative updates and services packs being applied on a server with this service pack installed.
We will have to wait and see what Microsoft advise going forward, I guess they will just release a new package that addresses the issue, lets hope sooner than later.