Ian, - not saying at all that your image is wrong and mine is right. I sort of expected that you had increased shadow detail, and its likely back where you started now, however I think you may have overdone the shadow detail retrieval and ended up with the plane looking "flat", which is how it appears on my screen. Dont know which method you used to do it, but if its the shadow tool in CS, a Radius of more than about 19% will tend to look a little fake and obvious. Flat means that items that are supposed to be black, are not black, but closer to grey, and if you take the tyre as an example, an item which should actually be black, it is not black in your original. Selectively adding shadow detail may also be a better approach rather than applying the tweak to the entire underside? If you simply open a curves or levels graph you will see the histogram is not even close to the black or white ends of the tonal range, which, even without looking at the image at all, indicates low contrast!
I was not aware that you had added the sky, - that would have impacted how I would do the mod, - so I will load another now that I know that. The plane will still be a little darker, but the shy will be as is from the original.
if you prefer the look you have achieved, with a lower contrast plane, you should be happy with it, - its no problem at all to disagree.
Also keep in mind monitor differences, - you should be using a calibrated monitor, and also your browser has to support colour management. If you use anything other than the latest Firefox or Safari 5 you do not have a colour managed browser, and will not see what I see.
Take a look at the new mod and let me know what you think