I started a new project today - MacRuby Document Based Application. I made the usual start up noises - git repo., etc. When I built the project just to check I'd done nothing stupid, I got 'failed - 33 errors' with the error above. (Thinks - it's going to be one of those days!) A quick check in Project Build Info showed the Base SDK set as Current Mac OS - Lion, on this box. I reset to Mac OS X 10.6 and the errors disappeared. I don't know if this is a Lion issue (accepting that it is pretty far from being released) but thought I'd warn people.