This works in Debian Wheezy to reverse the scroll direction (aka Natural Scrolling):

echo "pointer = 1 2 3 5 4 7 6 8 9 10 11 12" > ~/.Xmodmap && xmodmap ~/.Xmodmap

No reboot required. Reverting is as easy as running the command above again, but swapping the 5 with the 4 and the 7 with the 6.

Credit to Andy C’s Weblog