Got a W520 in a 170W mini dock. I've got two displays hooked up to DVI, and am using the W520's display as a third monitor.
The main display is a 24", running 1920x1200. It's in the center. The left display is the W520, and the right display is a 21" running 1600x1200.
If I minimize all windows, and place the mouse at the bottom of the W520's display and continue to pull down, it scrolls my desktop icons. The recycle bin ends up off the top of the main monitor. In addition, if I have a full screen remote desktop session in progress, the same thing happens and I see my entire remote desktop window move up.