OS X - Snow Leopard seems to have two states when it comes to the switching behavior. Dragging a window or not dragging a window. If you are not dragging a window, it ignores your mouse hovering at a side. This makes sense, or the dock and hot corners would be very hard to use. When you are dragging a window, it auto-switches workspaces if you hang around at a side. This also makes sense.
Sometimes though, it seems to get confused about if you're dragging a window or not. When this happens, even if you aren't dragging a window, it still flips workspaces if you hover the mouse on a side, thereby making the dock and hot corners very hard to use.
In order to reset this state, I figured out that you can pick up a window, drag it around a bit, then let go. This seems to reset its "you're currently dragging a window" state with respect to workspace switching.