Microsoft has just released SQL Server 2008 Service Pack 1 for download.  Packages are available for x86, x84, and IA64 installations and are each less than 400MB in size.  Most of the changes and updates are behind the scenes, but this is a fully-tested collection of post-RTM bug fixes.  eWeek magazine has some coverage on the update in Microsoft Releases SQL Server 2008 Service Pack 1.  From the article:

SQL Server 2008 SP1 contains a handful of enhancements, including Slipstream, Service Pack Uninstall and Report Builder 2.0 Click Once. Slipstream allows administrators to install SQL Server 2008 and Service Pack 1 in a single instance. The feature is designed to decrease the total time for an installation by, for example, requiring a fewer number of reboots, thereby increasing productivity.

SP1 also includes all the fixes and cumulative updates for SQL Server 2008 issued so far.