Also, don't forget about Diffraction in the megapixel war.
To quote directly from The Digital Picture (he says the same thing for every camera review):QuoteDLA (Diffraction Limited Aperture) is the result of a mathematical formula that approximates the aperture where diffraction begins to visibly affect image sharpness at the pixel level. Diffraction at the DLA is only barely visible when viewed at full-size (100%, 1 pixel = 1 pixel) on a display or output to a very large print. As sensor pixel density increases, the narrowest aperture we can use to get perfectly pixel sharp images gets wider.
DLA does not mean that narrower apertures should not be used - it is simply the point where image sharpness begins to be compromised for increased DOF and longer exposures. And, higher resolution sensors generally continue to deliver more detail well beyond the DLA than lower resolution sensors - until the "Diffraction Cutoff Frequency" is reached (a much narrower aperture). The progression from sharp the soft is not an abrupt one - and the change from immediately prior models to new models is usually not dramatic.
the 7D (& 60/600/550) has the highest pixel density of any canon so far, and the DLA is f/6.8. I know you can't really tell that from f/11 for most things, but i like to keep my f/ below about f/8 wherever possible, except for really deep DOF landscapes. look at almost any review of any lens with any sensor at www.photozone.de, every MTF chart they produce starts tailing off at f/5.6 - f/11 too, depending on which camera for the exact spot.
edit: example: the TSE-24 ii is arguably one of the sharpest around these days. check out the www.photozone.de MTF Chart. Diffraction is already hitting it between f/5.6 and f/8 in the centre, and between f/8 and f/11 on the edges, and that's tested on a 5d2, with a DLA of f/10.3.
I don't know the exact formula for DLA, nor how badly diffraction affects the picture quality per f/stop past the DLA, but this is another limit we're going to reach sooner or later.
What happens when we reach a DLA of f/2.8? we simply can't pack more pixels in, there'd be no point for anything except wide-open for fast lenses, slow lenses would be pointless. Given that all but the best lenses are sharpest around f/4-5.6 these days, we'll need ever-widening apertures and sharpness wide-open to get anything better than we have now.
just take again the 7D, upscale the sensor with the same density to FF, we get (5184/22.3)*36.0 = 8268 and
(3456/14.9)*24 = 5566, giving a total of 46 megapixels for the same DLA on a Full Frame.
So we've got a bit of a way to go until then, but don't expect wonders once the 40+mp aps-c cameras start rolling in unless you've got fast lenses sharp from wide-open...
Dont forget diffraction isn't so much an issue with the camera per se but the cameras resolution showing the flaws within each lens, hence why each lens has their own breaking point. Some of the older lenses 17-40, 16-35, 50mm 1.4 etc were created in the film days and were class leaders at that time. As digital cameras grew and grew and started outresolving film and showing flaws in the lenses, canon has been upgrading lenses based (probably) on sales volume and demand hence the 16-35 mii, 70-200 f4 IS, 70-200 F2.8 IS II, etc... While I slightly agree with neuro regarding the higher MP cameras the higher the lens total resolution, however i feel it isn't a direct linear curve reflecting the higher MP... The higher the resolution of the camera will degrade lenses.. some lenses will degrade quicker than other, however, it should be noted. For instance a 28-80 will have a quicker degrade and quicker diffraction point than lets say a 24-105 or 24-70...