I was curious if the feature was already in if not I think it should be.
Use ~ / make re-mappable and allow the camera to freely look about with normal human limitations and at same time make mouse movements no longer affect character turning, or something along the lines I think would be an entertaining feature in a game, and in general I hate getting blindsided in other first person slashers like Mortal Online or Mount and Blade (I don't use the first person.)
No ALT is the better key for this. Arma had it right! Its much more fluid than M&B (Referring to the ~ key)
In a first person game this would be valuable for the experienced, but I doubt most would use, being that there is not a huge need for it unless your traveling forward looking where your allies/enemies are. Its good for situational awareness.