It seems clear from our size-comparison exercises below that the upcoming Sony "Flagship," a.k.a. A900, is intended to compete with the Canon 5D (12.7 megapixels, moderate overall size, full-frame). That "class" of camera can't really be considered a class yet, because, for better than two and a half years now, the Canon 5D has been the only camera in the class. And yet, I think it's likely that this not-quite-a-class class of camera is probably the next locus of activity and progress in the DSLR Universe.
I suppose you could define the class as "cameras of normal size and very high image quality but without professional-level speed." Pros pay dearly for very high frame-rates, large buffers, bleeding-edge shutters, and so on. But many photographers don't need all that—to shoot a landscape, or a flower, or a portrait doesn't require things like SOTA focus-tracking or minimal mirror blackout. Just reasonable portability (and relatively reasonable cost) along with high image quality.
Market share is only one way of looking at a camera's importance. Another way is to look at significance, by considering a product's importance to real photographers relative to its sales and profits. By this measure, for example, Leica rangefinders have always been far more important than their market share would imply, whereas, say, Minolta's various pro cameras in the waning decades of the last century were one example of cameras that were less important to photography than raw sales figures might indicate. (The Minolta 9xi and Maxxum 9 were brave tries at full-on pro camera bodies, but I never heard of very many pros using them.) The Canon 5D has been popular among serious photographers, and I think its importance in that regard outstrips its market share handily. It's been a significant product.
By the time 2008 is over, we might well have full-frame, high-image-quality, moderately-sized "5D-class" cameras from several manufacturers. Nikon has a fairly obvious roadmap to get there: a "D3X," high-megapixel pro camera first, as soon as it won't poach sales from the hot-selling D3 (and maybe a little sooner), and then a 5D-competitor with the D3X's (or even the D3's) sensor in a smaller, slower, more reasonably-sized, and less expensive body. That's two generations hence, and there's no telling if Nikon will get there before the close of 2008. The Sony flagship / A900 will almost certainly be on the scene by then.
That leaves Canon. The 5D's 12.7 MP is no longer quite so special, since APS-C sensors have caught up with and in some cases surpassed the once-impressive magic number. The Sony A900 will almost double it. As to the second coming of the 5D, I'm not privvy to the schedule, but then, it seems no other prognosticator is either: like millenarians, some of them have divined deadlines that so far have come and gone without the desired apparitional event taking place. Still, it seems inevitable that Canon will at some point replace the 5D with a newer camera in the class the 5D invented and still defines.
It's just that the new camera will have competition. That will be a big difference. It seems likely to heat up this category considerably, finally, to the benefit of serious photographers worldwide.
Featured Comment by Colin Work: "I owned a Canon D30, which, though limited in many ways, produced the most beautiful images (at 100 ISO). But 3 MP was not enough, so I duly moved to a D60...10D, 20D, 1D Mk. II; but while each was a technological improvement, none captured the silky look of the D30. And then I bought a 5D, and I found that look again. My favorite DSLR despite (perhaps because of) its quirks and limitations. I also own a 1D Mk. III—it gets the job done, but the 5D makes me smile.
"I think the D30 and 5D prove, that for that special look, you must have big photosites. It is almost certain that the replacement will have more, and hence smaller, photosites. It will be better in many measurable ways, but I bet the images won't have that special 5D/D30 look to them.
"Sadly, being plastic and electronics, I doubt anyone will be shooting with a 20- (or even 10-) year-old 5D in the future as they might with a classic mechanical film camera. But as far as I'm concerned, the 5D is the first DSLR with 'soul.' "