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The flat subject parallel to the focus plane is perhaps the best way to illustrate the resolution fall off, if there is detail in a third plane then the possibility of field curvature begins to intrude.
Unfortunately Prime lenses are not immune to this either Canons 20mm f/2.8 is a particularly poor performer, but all lenses show it to a greater or lesser extent, and high resolution FF sensors make the problem worse.
Funny but when I posted this on one of the pro forums they had seen it and understood that with higher MP sensors comming the problem will be more noticeable as centre resolution will continue to rise while borders will stay broadly similar. Smaller resolution cameras such as the 5D MkI or the Nikon D3 don't show this effect so much as the resolution is maintained across the frame more evenly.
The real issue is that sensors have evolved much more quickly than lenses have, even recently designed lenses are showing this problem, maybe the whole DSLR system needs a rethink before it moves forward again
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