There seems to be no doubt that a 22mp full frame could be capable of providing marginally higher resolution in the central part of the image which is captured by the crop format cameras such as the 20D, 30D and Nikon's D2X. There have been so many comparisons on the internet which demonstrate the slightly higher resolving power of the 'cropped format' sensors with higher pixel density. The 1Ds2 has approx. the same pixel density as the D60 and 10D and any future 22mp successor to the 1Ds2 would have approx. the same pixel density as the current D30. The Nikon D2X with its 1.5 crop factor already has greater pixel density than a 22mp full frame.
We seem to have a situation where the D2X is considered to produce images at least as sharp as the old 1Ds and only a whisker behind the 1Ds2. That fact in itself would suggest that there are at least some lenses that will be up to the job of providing greater detail with a 22mp sensor. However, I would have to agree that for edge and corner resolution, 22mp would more often than not, be overkill.
But we should get this into perspective. Generally, edge and corner resolution do not carry the same weight as the central part of the image. Photodo ratings take this into consideration. They are 'weighted' ratings and a lens is not marked down as much for poor edge performance as it is for poor performance towards the centre. This policy of Photodo is presumably in line with common perception. Furthermore, I would suggest that photographers using shallow DoF in a creative way will often not be concerned at all about resolution fall-off in the corners.
Wide angle lenses are going to present a problem, but they already are a problem. When I got my 5D, I was a bit concerned that edge resolution at 15mm with my Sigma 15-30 would be a disappointment. Surprisingly, it wasn't. Nor is vignetting a problem at the small apertures I use for most landscape and architectural shots. What is much more noticeable than any resolution fall-off is the pronounced distortion towards the edges of the frame. This is a disaster if there's a person near the edge. Their body and/or head becomes very noticeably misshapen.
Bjanes has a point that greater pixel density will require the use of faster shutter speeds to make full use of those extra pixels. But this not as great a problem as one might think. A question that often cropped up in the past, on this forum, when people were struggling to get used to the implications of the 'cropped' format, was 'how is the 1/FL rule affected?' 'If I've been using 1/100th sec with a 100mm lens on my 35mm film camera (for a reasonably sharp hand-held image), should I use the same shutter speed with my 10D?' The answer was generally, no. The rule should be amended to 1/FL*1.6. That is, with a 100mm lens, one should use a shutter speed of 160th for a reasonably sharp hand-held shot, assuming we're talking about the same FoV and same print enlargement.
Of course, whether 1/FL is a good guide or not is another issue. Whatever your standards are for a sharp hand-held shot using a 1Ds2, then they would have to be increased by a relatively small margin. Roughly, I would say that 1.5x the shutter speed would cover it. If you were previously using 1/200th with a 100mm lens with a 16mp camera, then a 320th should be sufficient with a 22mp camera. Factor in the 2 stops of lattitude that IS provides and you're back to 1/80th. Considering the excellent performance of Canon DSLRs at high ISO (and I think there's no chance of them going backwards on this issue), an 80th is a very usable shutter speed for most lighting conditions, and if you think the performance of IS might be compromised with a higher pixel density camera and feel safer with a 1 stop advantage, then 160th would still be very usable.
Lastly, with many high tech systems which consist of separate components that develop at different rates (and Hi Fi systems are a typical example) it is not unusual for one part of the system to get ahead of another part such that the other part becomes the weakest link. When this happens, it is unreasonable to expect manufacturers of the stronger link to cease further development whilst waiting for the weakest link to catch up. We do not expect manufacturers of hi fi amplifiers to stop improving harmonic distortion figures because such improvements can not be fully reflected in even the best of loudspeakers.