I will use this post to answer all the questions I received regarding SP2013 SP1:
Question: Is March 2013 PU required to install SharePoint 2013 SP1?
Answer: No. SP1 can be installed as well on the RTM baseline and on the March 2013 PU baseline (means all CUs later and including to March 2013 PU).
Question: Which version is SharePoint 2013 SP1?
Answer: The official version number is 15.0.4569.1506. You can see this version number e.g. in Control Panel - Programs.
Question: Which version number will I see in the Central Admin?
Answer: The configuration database version showing up in the Central Admin is 15.0.4569.1000
Question: Do I have to install as well SharePoint Foundation SP1 and Sharepoint Server SP1?
Answer: No. SharePoint Server SP1 includes SharePoint Foundation SP1.
Question: Do I have to install as well SharePoint Server SP1 and Project Server SP1?
Answer: Yes. The Project Server SP1 does NOT include SharePoint Server SP1
Question: Do I have to run the SharePoint Configuration Wizard after installing SP1
Answer: Yes. The configuration wizard has to be run after updates, including Service Packs, CUs and Security Fixes.
Question: Does SP1 now support installing SharePoint 2013 on Windows Server 2012 R2?
Answer: The SP1 update which is currently available for download does not allow the installation on Windows Server 2012 R2 as it is not possible to create a slipstream installation for SP1. You need to wait for the release of SharePoint 2013 with SP1 included.
Question: When will SharePoint 2013 with SP1 (also known as "slipstream version") be released?
Answer: I will provide an update on my blog as soon as the updated version is available for download.
Question: I have SharePoint 2013 installed with 3 language packs. Do I have to install also SP1 for all installed language packs?
Answer: Yes. Unlike Cumulative Updates the SharePoint Server 2013 SP1 package does not include the updates for additional language packs. You need to install the service pack for each installed language separately.
Question: Where can I find the list of all Fixes included in SP1 since RTM?
Answer: There is no consolidated List for all Fixes since RTM. Here you can find all fixes which are unique to SP1. Fixes included in earlier CUs are documented in the KB articles of these CUs.
Question: Are there any known issues with the Service Packs
Answer: Ensure to read the KB article of each Service Pack carefully as known issues will be listed there. For SharePoint Foundation 2013 installations review this post: http://blogs.technet.com/b/stefan_gossner/archive/2013/10/26/october-2013-cu-for-sharepoint-2013-has-been-released.aspx