since a week or so (with the update to firmware version 3.0.0.5 ) the nintendo 3DS records and plays movies...
it didnt take a week to find a converter, that converts files to the 3DS format:
either a lil script:
http://socoder.net/?blogs=29026 (or http://www.socoder.net/index.php?blogs=29000 )
or a quick and dirty gui that does the same:
http://dl.qj.net/nintendo-3ds/applicati ... -beta.html

Hmmm. i really do not have 3d videos and am not able to rip em off yt, but i guess its workin fine \o/