Visual Studio 2010 has changed the way it builds application solutions. It has a new "build" engine that applies much more strict rules when building a solution. This issue is a know problem for many people who have converted over to VS2010. If you feel that your code is not in error, I would suggest you take a look at this. It should help you find why the compiler thinks your porjects are not up to date. Also, this explains what many people are experiencing with VS2010.
BTW...I currently have an open incident with Microsoft to resolve the same problem with my code.