According to the latest round of age ratings issued by Germany's USK board, Gran Turismo 5: Prologue (GC-Demo) has been cleared for all ages, meaning that Polyphony Digital's teaser release (which we expect to be released over here towards the end of this year) should be available in playable form at Europe's premiere games show.
While it's by no means confirmed, the obvious hope is that Sony will then release the demo on the PlayStation Store. While Sony has lagged behind the likes of Microsoft is making demos and trailers from major tradeshows available to download, if the demo isn't too big, it's not beyond the realms of possibility.