I just watched the awesome video[www.joystiq.com]
of this in action, and one thing seems to be missing: a view aim mode where your mouse controls the gun/hud, and head tracking simply controls the view direction (analogous to reality). It seems like this is the obvious way that VR should operate and it doesn't seem to be present in the Oculus Rift User Guide.
Scenario: if I'm looking in charging the enemy base as a heavy, I'll be looking in front of me (presumably at whatever I'm shooting at). But I may momentarily want to look around/over my shoulder for a medic or an enemy spy. I don't intend to aim in that direction; I just want to look around. That's the advantage of having hands that can point one way and a head that points the other way :P
In what way do you ever aim anything with your face, other than your eyes?
Maybe it doesn't work well in practice or requires some adjustment, but it seems to make the most sense...