I'm pretty sure I read of it on a web site lunk to from here, but I don't have a record of it. I have seen it reported from at least two different people who tried it, but don't have a record of where I read it. Sorry.

EDIT:

I've found one reference to a teacher who's trying it but has some kids who don't watch the videos:

http://www.msteacher2.org/forum/topics/flipping-the-math-cla...

Here's another in the comment stream for the Khan TED talk that suggests the idea:

http://www.joannejacobs.com/2011/03/khan-use-video-to-flip-e...

There are more, but they require filtering/finding:

http://www.google.co.uk/search?q=math+homework+flip+video+%2...

Thanks. That's somewhat disappointing, I guess.