At the world cup finals in Hafjell, Norway last weekend, the top guys were hitting 82 kmh (51 mph~) on a open section speed trap. They aren't quite as fast in tighter sections, but still plenty fast.

Here is the helmet cam footage of Josh Bryceland that probably would have ended up winning if he had not overshot the last jump and broken his foot. https://www.youtube.com/watch?v=XBDpu4-pQhM The speedtrap is at about 2:30.

Not at serious downhill events, a lot of protection is worn though.