SP1 of Reporting Services can be downloaded from the following site:

http://www.microsoft.com/downloads/details.aspx?FamilyId=580FEBF7-2972-40E7-BCCF-6CD90AC2F464&displaylang=en

It contains the following new enhancements and fixes:

• Excel rendering extension has been improved and now supports viewing in Excel 97 and 2000.
• PDF rendering extension is more robust and has better performance.
• Chart control provides more control over display styles.
• References to external URLs (images and resources) from within a report are now supported.
• Data caching behavior for report preview is now supported.
• Newline in expressions is now supported.
• The style of the HTML Viewer toolbar can now be modified through a style sheet.
• New URL parameters offer more options for customizing report presentation at run time
• Report Manager proxy persists authentication cookies so that they can be used by custom security extensions.
• Hidden parameters are now supported.
• Temporary snapshots can be compressed as well as stored on the file system.
• Integrated security support for accessing report data sources can be disabled.

The Readme file is a good place to start to get the latest, in-depth info on the above improvements (okay, I know nobody reads the readme, but I'm a doc guy and I have to plug it, and it will seriously be worth your time if you are interested in any of the SP1 enhancements).