VS2010 projects and VS2008

I just tried to open a c# project created in vs2012, .net4 with vs2008.

After changing the target framework to 3.5 everything was fine...until I tried to use some LINQ.
I was told me to add a reference to 'System.Data.LINQ'. So I tried to add a new reference but ... none of the 3.5 references were available (only 1.*, 2.* and 4.*).

Solution:
- Close your solution.
- Navigate to the solutions directory and delete the *.suo file.
- Open your solution.

Now, all the references of the .net versions (in fact the 3.5 ones) should be available again.

Edit: adding a 'using System.Linq' makes it complete