There were a couple of questions asked at last night's slightly chaotic SQL Server event in London (who set off that fire alarm?) on the subject of Reporting Services and moving from 2005 to 2008. One specific question was: "Can I take a SSRS 2005 report definition and open it / upgrade it to 2008?"
From the readme file for SQL Server 2008
"You can use the SQL Server 2008 designers to open and modify projects that were created by using the SQL Server 2005 designers. However, if you use the SQL Server 2008 designers to modify SQL Server 2005 Business Intelligence projects, you can no longer save those projects in the SQL Server 2005 format. Upon opening a SQL Server 2005 project, the designer upgrades the project in memory to the SQL Server 2008 format. To permanently replace the SQL Server 2005 version of the project with the upgraded SQL Server 2008 version, you must either save the upgraded version of the project or perform an operation, such as running an Integration Services package, that will cause the upgraded version of the project to be saved. Otherwise, the upgraded version will not be saved."
So yes, you can open 2005 reports in 2008 but they are upgraded on save and can then no longer be opened in the 2005 designer.