Vote for this bug if you encounter this problem.  Basically in 2010, if you regen the DBML file outside of VS the underlying DataContext code does not get regenerated.  This is bad because changes outside of VS 2010 to your DBML will not get ‘picked up’ when you rebuild.

https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=474117