Sadly, this is Microsoft's official position.
http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=2473242&SiteID=1
From MVP Teo Lachev: "VS.NET itself doesn't support BI project types. They get added to VS.NET when you install the SQL Server client tools. Since SQL Server 2005 client tools don't know anything about VS.NET 2008, nothing gets integrated with VS.NET 2008.
I believe that working with SQL Server 2005 projects will require you stick with BIDS 2005 or VS.NET 2005. That's because when you open a RS 2005 project in BIDS 2008, the report definitions will get converted to the new RDL format so you won't be able to publish them back to SQL Server 2005. I hope someone from the RS team reads and confirm this."
From Microsoft's Robert Bruckner: "In CTP November, BIDS 2008 is essentially equivalent to BIDS 2005 (based on RDL 2005).
However, this will change in future CTP's as BIDS 2008 receives all the updates for a new and improved report design experience. It will then also use the RDL 2008 format and upgrade old reports on loading into BIDS."
Conclusion: You need to keep a side-by-side installation of Visual Studio 2005 in order to open/edit SQL Server 2005 Reporting Services projects, until you are able to install the SQL Server 2008 Client components on your machine.