Wow ... what an over-the-top thread, and entirely missing the original question!
To cut to long story short---Morgan is right. With different image formats, depth-of-field sharpness indeed does change differently across the depth of the field. The gradient will be smoother with larger formats. However, with 35-mm format vs. APS-C, the difference is hardly visible (at long range even less than at close range); it'll be more obvious with more dissimilar formats like APS-C vs. medium format, or 35-mm vs. 4×5", for instance.
Depth of focus will be similar ...
Yes! It's over the top, but all my tests have shown that magnification is very similar. The issue has naturally gravitated towards the 'resolution' of that similar magnification. That's now become the issue, and it's the central issue which has flowed on from a beginner's question.
Morgan has suggested that comparisons will vary with focal length. I've tested the extremes of 250mm versus 400mm, and 18mm versus 30mm. I've now gone somewhere in the middle and compared 58mm on the 40D with 105mm on the 5D.
Surprisingly, the results are different. I didn't expect it and I was already composing in my mind some derogatory comments, before I saw the results.
In this range between 50mm and 105mm, with my
lenses at least, the results tend to suggest the larger format produces slightly less DoF for the equivalent FL and F stop.
Has this anything to do with the mathematical formula presented as evidence for this effect? Who knows? Once again, it all boils down to 'knowing thy lenses'! Because my Canon 24-105 behaves like this is no guarantee that all lenses will behave similarly.
Of course I offer the results at maximum quality jpeg comprerssion. First the over all scene.
Then for dedicated pixel peepers, a 100% crop of of the 5D at f8 versus the 40D at f5, with the same zoom lens. The 5D is marginally sharper and more detailed, but not by much. There are all sorts of issues such as, is the 24-105 sharper at 105mm than at 58mm? (The FoV's are the same but the nomenclature may be erroneous.)
Let the results speak for themselves. The image headings should provide sufficient information. You can see from the following 2 images that DoF resolution does in deed vary noticeably with the these focal lengths (but not greatly). Perhaps Roger Clark should re-examine his views.
The bottom left hand corner is slightly nearer than the plane of focus, yet it is still marginally sharper in the 40D shot, confirming that at these focal lengths, something different is happening. The larger format does indeed tend to produce a slightly shallower DoF.