The reporting services addin to that needs to be installed on the webfront end servers gave me some grief last week during installation. My customers installation environment was a x64 bit MOSS box, with SQL Server 2008 Entterprise edition on a seperate box. After sucessfully configuring the SSRS in SharePoint Integration Mode on the SSRS server, MOSS bits also need to be installed on the database server. Remember to use the same bits of MOSS (including Service Packs and patches) as used on other servers on the farm. Please do not install IIS on SQL Server 2008 (SSRS) as SQL Server 2008 does not require IIS, you only need to install MOSS without IIS.
The real issue I faced during installation was that of the SSRS add-in for SQL Server 2008 x64 ( different from one for SQL Server 2005 SP2) , was that the msi used to fail repeatedly and the following steps (From SQL Server BOL)helped fix the issue.
To get the path information for this file, click Start, click Run, and then type %temp%. The file should be located in \Documents and Settings\<your name>\LOCALS~1\Temp.
This configuration step will take several minutes to complete. The W3SVC service will be restarted during this process.