Here's a just published article by David Pogue, the NY Times tech guy, who explains the reasons why the more megapixels a camera has doesn't mean that its pictures are better....and that all this hype and spin about megapixels is nothing but a marketing ploy by the camera manufaturers to sell more expensive cameras, and to unecessarily accelerate obsolescence.
In a test, he and his associates compared large prints using the 16.7-megapixel Canon EOS-1Ds Mark II flagship camera in a studio, three photos of the same subject were taken at three different resolutions: 7 megapixels, 10 and 16.7. The results were virtually the same.
So are the photographs using the 16.7 mps Canon virtually identical to those using a Canon EOS 30D 8.2 mps? In my view, I think they would be very close...and almost impossible to tell apart unless the photographs are blown up to print sizes such wall-size retail displays where the 16.7 mps would have the advantage. However to state the obvious, the lens used on both cameras should be the same.
Read the article:
Breaking The Megapixel Myth.