Before installing SP1 for System Center 2012 Configuration Manager please read the following:
KB Article: How to upgrade System Center 2012 Configuration Manager to SP1 – describes the upgrade process.
Article: System Center 2012 SP1 Woes, Gotchas, and Workarounds – on myITforum, it is updated with the issues you may encounter after upgrading to SP1.
KB Article: Installation error 0x800b0101: System Center 2012 Configuration Manager Service Pack 1 client – describes the following potential issue: You may not be able to install the Configuration Manager Client. You need to install this hotfix after upgrading to SP1.
And of course, if you are wondering, What’s New in Configuration Manager SP1 and List of public KB articles and major fixes that are included in System Center 2012 Configuration Manager SP1.
Later edit: Configuration Manager 2012 SP1 binaries have been updated and re-published on Jan 25, 2013. If you downloaded SP1 previously than Jan 25, 2013, you need to re-download them. The new binaries contain the updated MicrosoftPolicyPlatformSetup.msi and the corrected Endpoint Protection binaries for Linux/iOS. The rest of the installation media has not been changed.