hello. i've recently become interested in this topic since my switch to Windows 7.
I'd like to add by offering my observations.
when playing games this occurs most noticeably. im running a multi-monitor rig; which with this issue, makes gaming pretty difficult.
What i have noticed is that the mouse barrier (invisible box around the screen; determined by your resolution) extends to the total width of the virtual desktop; as appose to the single screen. as you are moving the mouse along the x axes (left & right, to direct movement); it will most likely track to the second monitor; at this point you're a click away from focus being taken onto the desktop.
i've also notice that this isn't happening with the older games. in which total des where the mouse barrier is active on a single screen my research has attributed this to DirectX 11; but i believe i am wrong.
crysis 2 it happens, civ 5 it does not.
both native directx 11 games.
//edit: a buddy sent this along in response. http://www.dr-hoiby.com/MouseJail/