Am I missing it?

If it's absent, is there an ini file I can edit to change it?

Look in My Documents\My Games\Skyrim and open SkyrimPrefs.ini

Look for this line:

bInvertYValues=0

And change it to

bInvertYValues=1

Remember to back up the .ini file before changing it.

Thanks!

Turning off mouse acceleration really made the cursor move much better in the menus too.