There is a syntax error in the document
\railsprojectdir\vendor\plugins\backgroundrb\generators\worker\templates\unit_test.rb, refactoring is not possible.
Turns out, both unit_test.rb and worker.rb actually contain templates and not ruby code.
I did log a bug at the RDT Refactoring Trac and the guys there very helpfully suppressed the error message - the fix is available in their Cruise build 567 - but the problem actually lies in the Rails code with templates having .rb extensions.
As a temporary fix I've renamed unit_test.rb and worker.rb to unit_test.rbt and worker.rbt and modified worker_generator.rb - which contains the code which uses the templates - to point to the renamed templates.
I guess I'll have to log a ticket at http://dev.rubyonrails.org/ - but it seems to be down at the moment. Oh well...