GAC’d assemblies are not always included in a wsp build by wspbuilder. See
http://wspbuilder.codeplex.com/Thread/View.aspx?ThreadId=22446.
To counteract this, add the following post build events
ATTRIB -R “$(SolutionDir)[FolderContainingFeatureProject]\GAC\” /S /D
COPY “$(TargetPath)” “$(SolutionDir) [FolderContainingFeatureProject] \GAC\” /Y