This blog has some .ini changes that should help.

http://linearskillz.wordpress.com/2011/11/13/skyrim-pro-tips-that-help-with-mouse-control-and-physics-glitches/
That fix will only help you if you are running at a pretty high FPS. If you are getting lower FPS, there isn't much you can do. Your vertical response is always going to be *much* lower than your horizontal response (unless Bethesda can give us a fix for this).
Vertical response is tied to your frame rate, so the lower your frame rate, the more sluggish your vertical response becomes. Increasing the sensitivity will end up giving you a hyperactive horizontal response and maybe a normal vertical response. Alternately, if you don't care about vertical response and your horizontal response is too fast, try reducing the Look sensitivity in your game play settings.