I have just started to receive the following error when running my WatIn tests.
System.IO.FileNotFoundException : Could not load file or assembly \'
I always end up recompiling watin so it isn't signed, as I couldn't make any of the suggested solutions work either.
You can get the source here: http://watin.org/
Open the solution in the \source\src folder (there are VS2008 and VS2010 versions), open both projects and go properties -> Signing, uncheck 'sign the assembly', and build the solution.
You should get two DLLs in \source\src\Core\bin (WatiN.Core.dll and Interop.SHDocVw.dll) - just copy these into your project and they should work Ok.