Greg Gulliote (spelling?), who was a pro in the early '90s, claimed in a letter to Ride a year or two ago that he could do them, along with a bunch of other new tricks. It was never confirmed, and highly doubted.

On the other hand, at the rate that Scotty Cranmer (and plenty of other riders) progresses, I wouldn't at all be surprised to see it soon; according to several people, including Bob from Staff BMX, he's doing frontflip turndowns now.