Here are some links being passed around the blogosphere:

Overview of this feature by Kevin Hoyt
http://blog.kevinhoyt.org/2007/04/25/flash-cs3-export-motion-xml/

Creating animation in ActionScript 3.0
http://www.adobe.com/devnet/flash/articles/creating_animation_as3_05.html

Motion XML Elements on LiveDocs
http://livedocs.adobe.com/flash/9.0/ActionScriptLangRefV3/motionXSD.html

fl.motion package on LiveDocs
http://livedocs.adobe.com/flash/9.0/ActionScriptLangRefV3/fl/motion/pack...

Animator Class
http://livedocs.adobe.com/flash/9.0/ActionScriptLangRefV3/fl/motion/Anim...