I am upgrading around 100 projects from 2.0 and 3.5 and was wondering if there was a bulk update in VS 2008 that I wasn't aware of.
Doing some searching it would seem not
http://stackoverflow.com/questions/1914489/change-the-target-framework-for-all-my-projects-in-a-visual-studio-2008-to-3-5
One alternative would be write an application to add a few items to the csproj file.
(System.Data.Entity added to target 3.5 SP1)
http://msdn.microsoft.com/en-us/library/bb398197.aspx
In the end I did it manually