I hate to bring up an old thread - but to keep up the debate - I received a little promotional package from BMW today (I just bought a 2011 about a month ago). Anyway, on one of the pages of the 'history' book, it says:
"One of the most recognizable logos in the world, the blue and white BMW insignia is known as a roundel. It's the only automotive emblem to depict motion as it portrays the movement of an airplane propeller against a clear blue Bavarian sky. This hearkens back to BMW's early days as a manufacturer of airplane engines that set many altitude records in the early 1900s."