Service Pack 1 for SharePoint 2010 Products is Now Available for Download
Service Pack 1 includes stability, performance, and security enhancements that are a direct result of your feedback.
It is strongly recommended to install the June 2011 Cumulative Update immediately after the installation of Service Pack 1. The June Cumulative Update includes several important security and bug fixes that are not included Service Pack 1.
More information on the Product Team Blog http://sharepoint.microsoft.com/blog/Pages/BlogPost.aspx?pID=984
For a list of fixes and feature enhancements with SP1 refer to http://sharepoint.microsoft.com/blog/Pages/BlogPost.aspx?pID=973