I'm trying to determine how to open/edit existing SQL Server Reporting Services (SSRS) 2005 report projects (.rptproj) and reports (.rdl) with Visual Studio 2008, without having to install SQL Business Intelligence Development Studio (BIDS) 2005.
You cannot. Check this forum posting which has a reponse from Microsoft.
Yes, it was an active decision that the 2008 design evironments would not support continuous backwards compatability but would rather be a one way upgrade. This was not a casual decision and it is understood how it can be an impact.
You can read the entire posting for more details on why you are unable to use VS2008 for SSRS 2005 projects. Very annoying, but I guess I can understand why they had to make that decision. Just about the only reason I have VS2005 on my dev machine now is for Reporting Services projects since our company is not going to be upgrading to SQL Server 2008 anytime soon.