Yes, once driving to CO during the summer. I wish I remembered exactly when it was, but I saw 7-8 C6's in formation going through Arizona, all with a similar 'tag' on the passenger side door [they passed me while I was in the slow lane]. I believe it was during the summer of 04, well before they were allowed on the road. It might have even been the year before, but that seems even more far-fetched.
I wouldn't believe it if I didn't see it for myself. I had a few pictures but 3 years ago my camera phone was pretty awful so I never loaded them to the net.