So I put together this chart to show how much height of a printed photograph (300 dpi) you'd get per megapixel for a 4:3 ratio image:
When cameras used to sell with lower MPs (2-5 MP), the difference was significant. Nowadays, the difference is more constant per increase in MP.
Here's another, showing a photograph's height per MP. Keep in mind that you can double these heights (150 dpi) and still have a very good photograph.
I wanted to put these charts out here 1) because I was playing with Google's spreadsheet charts, and 2) to show that the average camera nowadays (I'll go out on a limb and guess it's around 7 MP) is more than enough to print big photos. Additionally, I'd like to further discount MPs' marketing privileges by pointing out that as MPs increase in cameras, the lenses are more or less staying the same: small and compact. Just because you're throwing more MPs behind the lens doesn't mean the lens is letting in better light.