Notice Kirchen in that picture is actually on the nose of the saddle, so he has slid forward quite a bit and this tends to roll your hips under.

I was fit last week by one of the foremost fitters in the country and we were talking about riding posture, and I asked him why you see so many powerful pro's with their backs rounded and their hips rolled under. His response was something to the effect of, "alot of riders still prescribe to the old school fit style and haven't taken the time to have a professional fitting by a trained exercise physiologist. You may be more powerful with a rolled back for a shorter amount of time, but day in day out a stronger core and hips rolled forward will make you much more efficient." THAT is what we're talking about here; efficiency.

All pro's are capable of putting out tons of power, but you'd be surprised how inefficient many of them were (the Schleck brothers come to mind, prior to their new fangled Specialized BG fits. Andy looks 100% better now) . Tyler Hamilton is another guy who I see all over the place with a poor fit, in fact most of the Rock guys look like crap on their bikes.

A few riders who make good examples of proper fit (from a hip angle POV)