Hi Joe,
Not sure if it's your case, but that might happen when changing the target SL version of a project when some cached dlls are still used.
Double check the version and the location of your referenced dll's.
If your target SL5, the ESRI dlls should be the version 3.0.0.388 coming from the Silverlight5/3.0 directory and the system dlls should be version 5.0.5.0.
Then :
- clean your solution
- remove obj and bin directories
- exit visual studio
- try again
Hopefully this will work :confused: