Comment 17 for bug 873482

I pushed the patched package to lp:~jconti/ubuntu/oneiric/xserver-xorg-input-synaptics/bug-873482

It's the basic idea, but there may be better ways of implementing it, and while it works for my setup, I can't really say it won't break other devices (although at the worst it should only fall back to the number of fingers sent by the kernel instead of using active_touches).