"Product: Microsoft Visual Studio 2008 Team Foundation Server Beta 2 - ENU -- Error 29109.Team Foundation Report Server Configuration: SQL Reporting Services configuration encountered an unknown error. Verify that you have sufficient permissions to configure SQL Reporting Services, and try again."
You can also get this error on TFS 2005, apparently.
My solution to the issue was pretty simple. I had installed Reporting Services first, and then Sharepoint Services (WSS 3.0 to be exact) afterwards. WSS 'hijacks' the default web site, which is where the virtual directories for Reporting Services had been installed, and so the TFS install couldn't find them. I reran the configuration for Reporting Services (in Start Menu in the normal place under SQL Server 2005 entry) and moved the directories (the configuration tool did not indicate there was an error at all, which was initially confusing) to the WSS enabled site, and the installation proceeded correctly.