Question from JAlan: "If you took two shots from a 12-MP DSLR (A700, D300, etc.) and stitched them together, would the end result be equivalent to the results from a single shot from the A900 (or the new Canon 5D Mark II), or is there something special about an image coming from a large megapixel single sensor?"
The answer is essentially yes to the first question. The sunrise panoramic I posted a few months ago is 14633 x 4323 pixels, or about 63 megapixels, two and a half times the resolution of the Sony sensor. As you say, there are some limitations to stitching, and naturally there are likely to be some camera-to-camera differences between any two given cameras in the quality of the pixels—you might call it "photosite quality"—so a stitched picture from one camera might not be precisely equivalent to a picture from another camera with a larger sensor. It might be a little better, or it might be a little worse. But close enough, at least in terms of resolution—especially if the pixel-pitch (i.e., photosite size) of the two cameras matches fairly closely.
Another thing not to forget is that digital generally only uses as many pixels as it "needs," and throws away the rest. That is, if you'll look at the small JPEG at the bottom of my post below—the park scene—it's downsized for the web to about 800x533 pixels. That's approx. 0.425 megapixels. Even though it was shot with the A900, that file in that form doesn't have any more resolution than a shot from a 1/2-megapixel camera would, provided the photosites and processing were of identical quality; the picture doesn't look any better in that application than a picture from a 3-MP or 5-MP or 6-MP camera would. Of course you couldn't blow up details to 200% to see more real information like I did with the distant doggie.
There is a limited range of uses where any sensor of any given size will "show its stuff," so to speak. If you start with a 24-MP file and throw away all but 6 MP—to make a small print, say—you're really not getting any advantage over the same small print made from a 6-MP camera.
Generally, as a standard seat-o'-the-pants formula, the way I think of it is that a camera will yield a close-to-optimum-sized print about as wide in inches as the number of megapixels it has, and it will up-rez very comfortably to about 1.5X that. You can certainly up-rez far past that, with very presentable, even excellent results, depending on subject matter as well as all sorts of subjective quality factors, down to your own taste in print quality; but you won't be seeing more real detail.
The megapixel race really came down to the fact that some photographers urgently needed to print bigger...and that included printing for reproduction. A handful of years ago, when the Nikon D1 was the default professional digital camera, I saw a sports magazine that was produced entirely digitially, and the double-trucks (full spread) pictures looked pretty bad, with lots of intrusive grain and noise and not enough detail for the repro size. Pros very urgently needed more pixels. Today's 10-, 12-, and 14-MP cameras look much better at that repro size.
So, these days, you can pick your sensor size based on the size of print you'll most usually be wanting to print with "best possible" quality. If you print 5x7's for putting in albums, a 6-MP camera will be fine. For most 13"-wide printers, the current crop of 12- and 14-MP cameras will be good matches.
I do always wonder about photographers who have to have the latest, biggest and mostest cameras, but who never print. That is, they store, view, and share their pictures digitally. I have a 20" monitor that displays 1680 x 1050 pixels, or about one and three-fourths megapixels. You might want to crop sometimes, so having a few extra megapixels might not hurt. But I keep trying to wrap my mind around why anyone who never prints their pictures would ever need a camera of more than 6 MP—really, 4 MP would be more than enough—and I can't come up with any rational reason—unless, of course, they just love to compare the resolution of their camera with other models in online forums!
By my formulas, the A900 is for people who want to print "best quality" up to 24 inches wide and up-rez occasionally to make prints up to a yard wide (or pros who need to shoot for repro much larger than that to will be viewed from various distances, like posters or billboards). If you never print as big as 24" wide, then you really don't need as much resolution as the A900 sensor gives you (although there still might be good reasons to buy such a camera—more about that tomorrow).
Personally, I love all sort of photographs, and get my head turned by good ones almost no matter what the format or final size, and I did become enamored two years ago of some wonderful big prints that master printer Paul Butzi made from some of my files. But traditionally I haven't worked in "wall pieces," and, really, the huge majority of the prints I've made in my lifetime are 10 to 12 inches wide, with the occasional "big" one going up to 15 inches wide. So, according to my own formula, a 12-MP camera would be the most sensible choice for me personally. Although the lure of the occasional really big print is still a Siren song.