On certain hardware, xinerama seems unable to correctly calculate the size of the expanded desktop.
In these situations the desktop will appear to be just a little wider than the left screen.
The mouse will move over both screens, but you are unable to drag windows past the mistaken edge of the desktop. Maximising windows on the right screen gives a sliver on the right of the screen.
Menu bars can still appear correctly on the right screen.
This appears to be common on hardware using the i810 driver. For example, dell laptops with intel 945GM graphics driver.
for more info, email kerry at mayes dot co dot nz.