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JDD brings up some good points in his post on the Canon 5D Mark II: most lenses can't resolve enough detail to take advantage of a 21 megapixel sensor.

I must say I've been relatively disappointed with the performance of my 24-70L and even, to a smaller extent, the 70-200 2.8L IS. The 300/2.8 is incredible, however, and the 50 and 85mm lenses I own (f1.4 and f1.8 respectively) I don't expect much from to begin with.

JDD's point is that a 16MP camera (or even a 12MP camera) with incredible high-ISO performance would have been much, much more useful than a 21 MP camera, and I agree. Though the 5D is a great camera, particularly at high-ISO, there's a lot of room to improve there, and blowing past the resolution capability of even the premium lenses just doesn't make much sense.

JDD raises a few other points, and I recommend you read the post too.

Despite all of the objections and rationale, I still plan to find a way to grab a 50D and a 5DMII… perhaps not this year, but sometime. And yes, my 5D is still for sale.

P.S. Only loosely related, but interesting.