Fun: Tachyons and Čerenkov Radiation
The Fun of Going Faster-Than-Light : Starts With A Bang.
To the extent I would have ever thought about it, by symmetry I would think that a tachyon entering a medium in which it moved slower than c would emit some kind of impossible-to-detect tachyon-y version of Čerenkov Radiation, but apparently it would emit real radition that we could detect. Sweet!
Uh... But wouldn't that allow us to send signals faster than c?