What about this video?
http://www.youtube.com/watch?v=-_j2nX1Gc4Q

It looks really good and it should work with N900, too! The autofocus noise isn'is really quiet and if there are other louder noises like speech you won't hear it. AND, N900 has USB and an open source OS, so you could use an USB micro with some effort, so you could prevent recording the noise anyway...

I really hope video autofocus could be done by software and N900 is powerfull enough to do it.