You're right, you're back is not supposed to be completely straight. The proper wording is that your lower back would be straighter if your hips are rolled farther forward (and doing what Danno instructs).

Compare the photos that Danno posted to the photos I did. You don't see any difference in the amount of curvature in the lower back? Maybe I'm missing something then. It appears to to me that a lot of riders plant their butts like they're on a barstool and bending their lower back and hunching over to make the most of being layed out and producing power - even when they stand to sprint.

After danno's explanation I finally gave it a try on the way home yesterday. Very interesting. My lower back seems to like it much better and all the ab work I have been doing this winter seems to be helping.
Also, using the drops seems much easier. This seems to have improved my aerodynamics on the hoods as well.
Now to make it a habit.

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)

Exactly my experience last spring...changed out saddle, seatpost, stem and bar and ended up too far back from my previous position. Lots of lower back pain, especially when the road tilted up until I had my LBS re-do my fit.

I think you guys and gals are talking at cross purposes and advocating the direct opposite, no? Machka is talking about rolling the pelvis backwards, increasing the arch in the back, and Danno and others are talking about tilting the pelvis forward, bending at the hips instead of the spine.

Or am I the one who is confused?