After connecting an external monitor, the touchpad starts moving much faster in the horizontal direction than in the vertical direction, as if the screen area had been stretched rather than extended to include the area of both monitors. This is really disorienting, throws off muscle memory, makes it hard to move in the right diagonal direction, etc. Reproduced in both Ubuntu natty and Oneiric.

This bug was originally reported in Ubuntu on launchpad BTS:http://pad.lv/825548