This link is quite old, but I thought I would share it anyway.
I think this has alot of potential, but it doesnt seem like Autodesk has been implementing this into any of their packages yet (as far as I know). Does anybody know more about this?
have a look at CATIA Natural Sketch on YT! Seem's like Dassault has implemented such stuff!
Also have a look at http://www.ilovesketch.com/ - but you can't get it:( Only look do not touch!