My impression of Combat Expertise is that it is simply a representation of unusual parrying ability, which would appear to suggest that it should only apply against attacks that could sensibly be batted away. The definition of "sensibly batted away" is very ambiguous, especially when dealing with charging fiendish girallons and such!
I could see Combat Expertise being only marginally useful against missile attacks. Parrying arrows is difficult, and I would be inclined to say the ability to apply the AC bonus to missile weapons would be another feat. I'd even go so far as to call this other feat "Deflect Arrows" (which tells you what I think of the PHB feat of that name).