Taken from the download site:

SQL Server 2008 Service Pack 1 (SP1) is now available. You can use these packages to upgrade any SQL Server 2008 edition.
Note:We remain committed to our plans to keep service packs contained, focusing on essential updates only, primarily a Roll-up of Cumulative Update 1 to 3, Quick Fix Engineering (QFE) updates, as well as fixes to issues reported through the SQL Server community. While keeping product changes contained, we have made significant investments to ease deployment and management of Service Packs:

• Slipstream – You are now able to integrate the base installation with service packs (or Hotfixes) and install in a single step.
• Service Pack Uninstall – You are now able to uninstall only the Service Pack (without removing the whole instance)
• Report Builder 2.0 Click Once capability

Download it here.

Grz, Kris.