I know what this is: all the U.K. and Oz mudcatters have conspired to try to "April Fool" the rest of us into believing that sane adults actually do something that silly over there.
I never happened to hear of the Volkswagon Beetle thing for kids until a few months ago. The version I heard is, spot a Beetle, say "slug bug", punch the other kid on the shoulder, and try to continue with "no slug back" before he can respond. It's been adapted to "Cruiser Bruiser" for sighting P.T. Cruisers.