I've only seen them once on twice in photos and in person. These are the tire covers that cover the tires and part of the wheel while the wheels are still on the bike. They have "Shimano" all around them.
Sorry for the lame description. Here's the best photo I could find of something similar:
I have some somewhat delicate tires on my track race wheels that shouldn't be on the ground during transport through parking lots and such.
Anyone know what these are called? Any idea where I can find them?