I've done 53mph, off-road, back in my young and stupid years. Almost wrapped the bike around a tree at the bottom of the hill.
Well, if you want to include uncontrolled attempts, I'm sure I could achieve some pretty impressive feats.
Back in the early 90s, John Tomac hit 65mph on a fully rigid mtb in the Mammoth Mtn Kamikaze. I don't know what the speeds are these days, but I believe courses have been redesigned to bring speeds down to a reasonable level. With current improvements such as suspension and disc brakes, breaking 65mph that same course would be trivial IMO.
Chris