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