Resolution

This is a nice easy one, as the 5D II does what it says on the tin. Having 1.6 times as many pixels gives you a corresponding amount of extra detail (with good lenses, at least).

Above is the skyline at Canary Wharf, and below are 100% crops from the 5D and the 5D Mark II of the same area.  This illustrates how much bigger the 5D files are and how much more detail they capture.  "Per pixel" sharpness looks about the same to me, and I think there is plenty more detail to be extracted from this lens (Canon 200mm f2.8L at f8 on a big Manfrotto tripod) by even higher resolution sensors.  In both cases I used RAW files, processed with the version of DPP which came with the Mark II, with both types of noise reduction turned down to zero and sharpening turned up to 50.

Below are another two crops from the same image which illustrate the same thing.  Note the colour moiré in the 5D shot (top) in the brick pattern which is absent in the Mark II because the pattern is better resolved.

This result is not surprising and reminds me of testing the 20D against the 5D.  The 20D, with its higher pixel density, squeezed a lot more detail out of a given crop from the 200L.  The Mark II has pretty much the same pixel density as the 20D, so any lens which was good enough to resolve more detail with the 20D than the 5D will do the same on the Mark II.

You don't need low ISO, tripods, ideal conditions and an "L" prime lens to get an advantage from the extra resolution, either.  I won't post noisy pictures of the inside of my living room but messing around with the old consumer grade 28-105 f3.5-4.5 wide open, both cameras at 1600ASA, shooting JPEG rather than RAW and hand-held, I was able to see significantly more detail in the Mark II results. 

The answer to the question "which lenses are sharp enough to render more detail with the 5D II than they did on the 5D" is "all of them (unless they are broken or Tamrons which look broken but are proven to be "within spec")."

The answer to the question "which lenses are good enough that even the 5D II will never show up any optical imperfections" is "none of them."  The 200L is my best lens but even that sometimes showed significant chromatic aberration on the 5D, never mind the Mark II. 

I have also done some tests, which I think are too boring to post here, to see whether SRAW1 (9.9 megapixels) on the Mark II is as good as full resolution 12.8 megapixel RAWs from the 5D.  One theory is that because 1/3 of the information in a 5D 12.8 megapixel image is made up (interpolated), a 9.9 megapixel image which has been downsampled from 21 megapixels may actually contain more detail than a native 12.8 megapixel image.  It doesn't.  If you are planning on buying a 5D II and shooting SRAW all the time, don't bother - get a 5D instead and shoot RAW and you'll get better resolution.