Adding to Spesh's bit about time limits, the Commissaires can, at their discretion, reinstate riders who have finished after the cutoff time. Normally this happens when the autobus consists of so many riders that there wouldn’t be much of a race left if they were all sent home, but occasionally they'll let one back in if he’s been unlucky. Rick Zabel missed the cutoff by 3 seconds one year but they let him back in because he'd had a mechanical on the last climb. Current rules state that reinstated riders lose ALL* their points in the, er, Points and Mountains competitions, so were it to happen to Cav, someone else would win the green jersey this year even if he subsequently made it to Paris.
A pedant notes that while His Eddyness was KotM in 1969 he didn’t win the spottyjumper as the jersey itself was only introduced in 1975.
* Although TV's Matt Rendell was unsure about this the other day – unusually, as he knows everything about the history of bike racing – everyone else seems to think this is the case.