You must have two separate solution files. Visual Studio (any version) tries to upconvert any solution file to the next version based on the version inside the solution file e.g.
Solution File for VS 2010:
Microsoft Visual Studio Solution File, Format Version 11.00
# Visual Studio 2010
Solution File for VS 2008:
Microsoft Visual Studio Solution File, Format Version 10.00
# Visual Studio 2008
Solution File for VS 2005:
Microsoft Visual Studio Solution File, Format Version 9.00
# Visual Studio 2005
The same goes with project files, so you will have to have separate versions of each project file. Each version of Visual Studio have their specific needs. E.g. for a Web Application Project Template in VS 2010 there are web config transform files that make no sense to earlier versions of VS.
Bottom line is it is possible to maintain different solution and project files in Visual Studio 2008 and 2010. Visual Studio 2005 is out of the question since there is no multi-targeting support and you are using 3.5 of the Fwk.