MS Flight Simulator X is vert -, so I think we can be sure that good WS support is not a requirement for the "games for Windows" label.
Vert- can be ignored as a negative for flight sims fortunately, especially those with TrackIR support, since you can change the FOV at will, among other things. With FSX and Lock On, there's also 6DOF with the TrackIR so vert- seems more a concession to the fact it's easier in some form to spend more time on things like solid TrackIR support.
That said, for anything but flight simulators, anything less than native Hor+ widescreen support is disappointing. And I got a good chuckle out of seeing that Games for Windows on my copy of Company of Heroes. Nice that it supports Hor+ out of the box as well, although it's a shame that widescreen support isn't a stipulation for certification :(