Fred255 mentioned, TidBITS. 

Interesting ... until I read this statement,

" Since no photograph of any size can contain more than 3 to 4 million
elements of information, even when made from film, any substantial
enlargement needs to be composed primarily of pixels that do not exist
in the original".

Let's see, DSLR cameras are available with about 12 megapixels; I saw
a report of a concept DSLR camera that will have 50 MP. 

I would have to say there is no inherrent upper limit on the number of pixels
in an image and that the first part of his statement is false. 

As to the second part ... yeah, if you shoot only a 4 MP image, crop and
enlarge ... then photoshop will (probably) interpolate some pixels. 

OTOH, if, as an example, you shoot a 12 MP image, crop and enlarge, you
might not need to add any pixels. 

Comments?  Please?

--
Martin