You cannot use the same solution file between 2008 and 2010. Each version of VS expects its own version of the .sln file. You can manually update 2010.sln by opening it in Notepad and rewriting it.
Microsoft Visual Studio Solution File, Format Version 11.0
For
Microsoft Visual Studio Solution File, Format Version 10.0
It works only for solutions that do not use any new features from VS 2010, and as soon as you try to open the file again in VS 2010, it will have to update it.
The best way is to use a separate .sln file for VS 2008 and 2010