A lot of people have a tendency to print at a preferred original ppi (such as 300 or 240) and see more MP's as producing bigger prints. A similarly large group of people (with probably a large overlap) feel that 100% view on screen is the only way to compare captures from different sensors. A lot of folks who routinely print, don't care about any of this until they see the print 
What gets lost in this is the increasing degree of enlargement from the original optical image falling on the sensor and revelation of blurs/flaws/user errors/camera shake/technique/AF success etc etc( as well as a redefinition of DoF).
If we take an old "full-frame" 35 mm camera and make an e.g. 8"x12" print from it, we have enlarged the image about 8x. If we print at 300 ppi without resizing/scaling, the following table shows the degree of geometrical enlargement from the original sensor dimensions. Obviously the total MP will determine the actual size of the print, and the format will determine how much cropping occurs, but that's not the issue here 
Imagine a lens on a tripod and a succession of cameras attached to capture an image and then printed as we attempt to compare them.
It's not difficult to see why the full frame sensors (in bold) (as well as the 1D"classic" and D30) are so beloved : they don't get magnified as much, so all those flaws are less obvious.
Comparisons at the same print size mean that values in this table are irrelevant, and the MP count of the sensor will (largely) determine how big a print can be made before it looks inferior to the others in such a comparison. Comparisons on screen at 100% will be comparing "per pixel" quality differences and simultaneously examining the images at different enlargements.
None of these concepts is new and I don't have any axe to grind here, but I've not seen a table like this before, so I thought it might be useful as a reference in discussions on sensor comparisons.
- Camera_____Enlargement at 300 ppi*
- (35mm_______________8x)
- 1D _______________7.3
- D30_______________8.1
- 1Ds_______________9.5
- 1D2, 5D______________10.4
- 10D, D60, 1D3, 1Ds2___11.5
- 20D, 30D, 1Ds3, 5D2___12.5
- 40D_______________14.8
- 50D_______________18.1
- 7D _______________19.3
