One of the quirks I've run into while using WSPBuilder for my SharePoint projects is that occasionally a project will try and add SSOCLI.dll to the WSP. Because this assembly is not strong named the creation of a SafeEntry will always fail.
This doesn't effect the package or deployment, but it does annoy someone who is particular like me ;) I found an article that talks about an easy solution until the WSPBuilder gets updated to ignore non - strong named assemblies.
Just add the following snippet to the Post-Build event in your project, and it will delete the DLL and allow your WSP to be built without getting an error message from the SSOCLI.dll file because it's no longer there :)
del "$(TargetDir)ssocli.dll"