I tried the exact same thing with the exact same results.
When I compile the 35 project in VS2010, however, the compiled DLL works just fine. It seems like something is not compatible between VS2008 and VS2010. A DLL compiled in VS2010 may still be referenced by a VS2008 project as long as the framework 3.5 is selected. Interesting- (and glad I am not the only one who saw this)
-Mike