If you have installed ‘xmodmap’ you may add the following command in order to have reversed scrolling experience like in OS X Lion:
1 | echo "pointer = 1 2 3 5 4 6 7 8 9 10 11 12" >> ~/.Xmodmap |
The next time you start ubuntu gnome desktop it will ask you, whether it should load the xmodmap (you should add the .Xmodmap file). There you go.