One interesting consideration is that NOBODY has introduced a high-end DSLR lately. The last ones with any real innovation were the Nikon D200 and Canon 5D (almost simultaneous in late 2005). Both are wonderful cameras, but one would assume that, in a technology-driven market, someone would have updated something since then (if not those bodies, which are due, but not overdue, for an upgrade. then one of the older pro models - Nikon still has the D2hs in their line, which is pretty much a 2003 model). Canon's flagship 1Ds mk II is a 2004 model, and the 1D mk III looks like a pretty darned incremental upgrade to me, at least as far as image quality is concerned. The 2004 model D2x is also looking ready for an upgrade, not to mention that Canon still has a 2004 model with a mild facelift (30D, which is really a 20D) in the midrange.
These companies are smart, and they have a lot of folks designing cameras, so the fact that the only 2006 or 2007 pro DSLR so far is a 10 MP Canon that mainly fixes user interface annoyances got me thinking... All these 22 MP and higher ubercameras rumored for years, yet none appearing... Have we hit a fairly fundamental limit? Will adding pixels now either out-resolve our lenses or inevitably add noise and subtract dynamic range? It seems like pixel pitches have settled down in the range of 6 to 9 microns (with the D2x pushing a little bit, to 5.5), and this is not only true of 35mm type cameras, but the MF backs as well. The first 6 micron cameras showed up in 2004, and resolution hasn't gone up since then - one would think that, if a 4 micron camera actually took better pictures, we'd have seen one by now (and a high-res D3x would have to be, if it wasn't full frame). The chip can certainly be built - 4 microns is still huge by the standards of feature sizes found on other types of chips. A 6 micron full frame camera is a 23 megapixel body, but that may be the limit of where we can go with today's Bayer sensors.
It is also interesting that Canon hasn't introduced a 23 mp camera when they have BOTH 6 micron sensors and full-frame sensors - just not a 6 micron, full-frame sensor, but that should be easy if they have both parts. The fact that Canon didn't upgrade the 1Ds at PMA makes me think there's some technical hurdle, whether they don't feel that the 6 micron sensor (which would be a full-frame version of the Rebel XTi sensor pitch) offers professional image quality or it's just too much of a challenge for the lenses.
The wild card here may be whether something other than a standard Bayer sensor can offer better quality, assuming that we've run out of room on Bayer sensors (yes, digicams use 2 micron sensors, but they're plagued with noise and dynamic range issues, plus their lenses don't need much coverage). Fuji's SuperCCD and some variant on the Foveon sensor seem like possibilities...