I agree that the megapixel race has gotten to be nuts. The only reason for more pixels is to print bigger. My Nikon D200 SLR and Canon iPF 5000 printer are well matched to each other (with careful interpolation, I churn out stunning 16x24s from the D200). This suggests that cameras above the 10-16 megapixel range (a slightly higher pixel count will mean less interpolation, which is nice) will only make sense with 24 inch printers (or heavy cropping)... 17 inch printers already weigh up to 100+lbs and take up a huge amount of space, and 24 inch printers are twice as large, twice as heavy and more unwieldy to operate (the DesignJet 130 is the notable exception). 17 inch printers already have a minimum print size of 8x10, refusing to make 4x6s (except for the Epson 3800, which doesn't use roll paper, a staple of big printers). Most 24 inch printers will only do 8x10 if hand-fed one sheet at a time - they're really roll-only devices for all practical purposes!
The absolute limit of what's needed to feed a 17 inch printer is a good 34 MP file (that would be 16x24 at 300 dpi with no interpolation). 10 MP works darned well (it's about 180 dpi, although I interpolate up to print), because of viewing distance. A 16x24 inch print is big enough that you don't look at it from 6 inches away. If this race continues, we'll have that 34 MP file, then we'll blow right by it in a couple of years, ignoring the fact that the new cameras are producing unprintably large files (assuming they are producing GOOD files that size)!
This has already happened with point and shoots. 10 MP is too big for an 8x10 print (I'm assuming that most 13 inch and wider printers are owned by SLR owners). The camera manufacturers keep turning out 10 MP point and shoots, not publicizing that you can't even print all the detail without a specialized printer that costs more than the camera. Worse yet, the 10 MP sensors are noisier, more ISO-restricted and have worse dynamic range than 6 MP sensors. For the printers they're used with, the latest point and shoots are over-resolution, and in pursuit of that extra resolution, they've actually LOST image quality in other areas.
Hopefully this will all shake out to something like:
Point and shoots - 6 to 8 MP - matched to 8 inch printers
Consumer SLRs - 6 to 10 (better, lower noise) MP - matched to 13 inch printers
Higher end (Nikon D200, Canon 5D) SLRs - 10 to 16 (or 20) MP - matched to 17 inch printers
A few ultra-high end SLRs (Canon 1Ds mk III) - 20 to 30 MP - matched to 24 inch printers
Medium format backs - 20+ MP (emphasis on low noise, wide dynamic range) - unlimited print size (due to viewing distance).
The only way to view a 100 MP file is in sections - zooming in on pieces of the image. Fine for the Gigapixel Project, but who'd want to look at "Moonrise, Hernandez New Mexico" that way?