This is good to know, strange solution but it works:
http://social.msdn.microsoft.com/Forums/en-US/linqtosql/thread/bc2fbbce-eb63-4735-9b2d-26b4ab8fe589
I created an Interface that my Linq to SQL classes implement so that I can create a generic function that will work across a few tables. When I used the double equals (==) instead of the .Equals() in my LINQ query I got the runtime exception:
The mapping of interface member [...] is not supported.
Changing my query up to use the .Equals() it runs beautifully.