[iridium]

Three finger scrolling works on a mac (Lion). I think you can enable drag lock on snow leopard for the same effect.

EDIT: The mouse does move off the view port during drag, so its probably not that much more efficient.

[Camillo]

Look at the Trackpad page in System Preferences. There's a reason why it's called "Three finger drag" instead of "Three finger scroll". :)

[jrajav]

In Lion, I'm pretty sure this only works if you enable "Three Finger Drag" in the trackpad settings.