To an attacker, there's no appreciable difference to using a Knoppix disc than an official OS X password reset disc, so your suggested change would not prevent any but the dumbest attackers but would make genuine users' lives harder. If you've not read it, you should get a copy of Beyond Fear (http://www.schneier.com/book-beyondfear.html).

A tidbit I forgot to tag on previously - most users are prolly more worried that if someone has physical access to their computer that they're going to steal it than root it.