Related link: http://www.symbianone.com/content/view/1837/
Sounds pretty simple eh? Its not rocket science but its definitely pretty cool. Watchtone Studio from Makayama lets you convert existing video files (such as AVI, DV, MPEG) to a watchtone. A very easy to use application, that lets you create a watchtone in 3 clicks. A free trial is available from http://www.makayama.com/watchtones.html
If you need a development tool to help convert your nokia videos to a supported format the multi-media converter from forum.nokia will get you going… although getting ahold of an MPG shouldn’t be too difficult these days.