With a dual-head setup with separate screens (with or without xinerama) the pointer when controlled with the ALPS trackpad on my laptop can move to the second screen, but not back to the first screen. The cursor can be moved back to the first screen with a USB mouse.
This downstream bug has more information:
that bug was originally filed against the nvidia driver, but I have ATI, so problem is either in the server or input driver, I assume.
Server bug, affects all absolute devices.
Problem is simply that when we scale an axis range into [0..screen width], the latter is actually one past the highest possible coordinate and triggers the screen crossing. 0 is still part of the screen and does not trigger the screen cross. Hence you can only change screens to the right/bottom.
Fixed with the commits leading up to
Author: Peter Hutterer <email@example.com>
Date: Tue Oct 4 11:41:17 2011 +1000
input: change pointer screen crossing behaviour for multiple ScreenRecs
This is a large change, so I don't expect it to be back-ported to 1.11.