A reader writing into the "Digital Photo Newsletter" (Vol 7, No 6) suggests that camera manufacturers are being as misleading about megapixels as with digital zoom.
(We know that digital zoom is not as good as optical zoom, but that some digital zoom, like up to about 4x, is okay. It's when the camera boasts 40x or 200x digital zoom that it becomes misleading.)
Anyhow, Gene Widenhofer suggests that we use the square root of megapixels to compare the resolution between digital cameras. Why square root?
Because megapixels refers to the area of the sensor. And the square root gives you the length along one edge of the sensor. And by comparing lengths (instead of areas) you get a better idea of the difference in effective resolution.
Here's a chart to show what he means:
Megapixels - Square Root
2.0 - 1.4
3.0 - 1.7
4.0 - 2.0
5.0 - 2.2
7.0 - 2.6
8.0 - 2.8
11.0 - 3.3
Using this chart, you see that the resolution doubles by moving from a 2-megapixel camera to an 8- megapixel model.
He argues that a more powerful optical zoom is better than a higher megapixel count, saying a cabin with more lake frontage is more valuable than a cabin with more acres. He doesn't mention the Canon S1iS, but that's exactly why I bought it: the 10x zoom is more valuable to me than megapixels greater than 3.