Canon seems to be headed down the path of adding megapixels with every new camera. Nikon seems like they don't care. That got me to thinking....am I limited by my low (10 and 12.8 ) megapixel cameras?
So I grabbed my kids and forced them to participate in a test shot (please ignore the un-combed hair and food stains). This test shot is with a 5D, 12.8 MP, 135mm lens and f/11. I then resized and uprezzed the shot to 20" x 30" at 240 DPI and had a 20x30 print made at Costco ($8.99). The linked image is not the uprezzed one because that sucker was 28 megabites.
Looking at the print, I'm hard pressed to note any lack of detail among the rear three kids. In fact, the biggest technical flaw (aside from the darkness of the rightmost child and the un-combed hair plus food stains) is that the little girl closest to the camera is slightly OOF at this large of a print size. DOF at f/11 seems to be almost inadequate to make a print this big.
Really then....I find f/11 for this little group of kids is the limiting factor. How then is 21 MP on the 5D2 or even more on the 1DsIV going to be of a huge value to most photographers? I can understand that a few people in real niche work might be able to use it, but I'm questioning if more pixels do anything for most of us beyond slowing down out computers and filling up our CF cards and hard drives.