Square inches have nothing to do with this at all
I've been a photographer in industry and done prepress work for 30 years. And I don't think this thread is a shining example of the public's general knowledge of digital photography. Some of it is quite revealing, and not always in a good way. Some information is so wrong it's difficult to know where to start. What with confusion over compression, pixels and bytes, resolution and definition, and now a camera's actual pixel rating! Oh, and terrible spelling here and there. But I think I'll stick with the so very wrong mega pixels per square inch.
Right. The pixel rating of a digital camera relates to the number of recordable pixels on the camera's array. In other words the potential number of pixels in the entire image. If the camera rating is 12 megapixels, that's it for the entire image. It is most certainly not per square inch. The number of pixels per inch becomes more or less according to whether you enlarge or reduce the physical size of the image in editing.
For example, an ideal pixel rating for a 10"x8" image for reprographic reproduction (300 pixels per inch) is about 8 megapixels, assuming no cropping. Less than 8 megapixels would mean that data would have to be extrapolated (created from surrounding data) in order to fill the 300 pixels per inch requirement.
If the camera rating were per square inch then one would expect to see another camera rating relating to maximum physical image size. There isn't one for the obvious reasons explained above.