The latest version of Mac OSX (Lion) introduced a feature referred to natural scrolling. Since I've been getting a few requests to add the same feature to the synaptics driver. At it's base natural scrolling is an inversion of the scroll direction*. We've had the ability to do this for years, though more by accident than intent.
The synaptics driver (until X server 1.12's smooth scrolling) uses buttons 4/5/6/7 for up/down/left/right scrolling. If you want up to scroll down, just swap them and you're good to go.
xinput set-button-map <device name> 1 2 3 5 4Now down is up and up is down. The device name can be obtained as usual with xinput list. This will work for servers up to including 1.11.
As of synaptics 1.6RC3 (220.127.116.113), the options VertScrollDelta and HorizScrollDelta can now be set to a negative value. For servers supporting smooth scrolling (X server 1.12+), this effectively inverts the scroll direction.
* I realise that natural scrolling in OSX may have a few other details to make it do whatever it does. Synaptics supports scroll inversion and scrolling inertia.