This recently released KB article http://support.microsoft.com/?id=918481 has some great information installing MOM Reporting on SQL Server 2005. It coverers a fairly high call generator to PSS "Unable to contact the Microsoft SQL Server Reporting Services server via Web Service" error during install. This can be due to NTFS permissions on %WinDir%\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files. See the article for more details.
Another commonly reported issue is "Unable to contact the Microsoft SQL Server Reporting Services server via Windows Management Instrumentation (WMI)". Please see http://support.microsoft.com/?id=918712 for details.
Please feel free to comment if these articles contain any inaccuracies or there are additional details we could add.