After upgrading to Ubuntu 12.04 I had this annoying issue with multiple monitor support. When I would drag the mouse from one monitor to the other, the mouse would stick on the Middle Dock for a few seconds before it would pass to the other monitor. To fix this you can turn off the "Sticky Edges" setting in the Display setting page.

After you set it to off you will need to log out and then log back in for the setting to take effect.