I had the same problem after closing a VLC session. With none of the dconf solutions working. The 'trackpad' was dashed as enabled, but was actually not working.
I was under 12.04, and upgraded to 12.10 but still had the same issue.
I recovered the trackpad with the
proto=imps option of modprobe:
sudo modprobe -r psmouse
sudo modprobe psmouse proto=imps
(c.f. http://superuser.com/questions/384885/ , no idea of what the proto option does!)
but then the 'two-finger scrolling' went 'off', and the 'trackpad' tab of the
'mouse&trackpad' entry in 'System Settings' just disappeared.
And still, the dconf editor says that the trackpad scroll method is two fingers...
I'll unfaithfully reboot ...