The 1DIII is a great camera, and Canon is to be commended. But I'll
point out that Nikon still gained worldwide market share over Canon
over the last year. So maybe things aren't as lopsided as you'd
like to believe.
Fluctuations in the low end are fairly easy to instigate. When Canon introduces the replacement to the 30D, we'll see a lot of sales volume shift back in Canon's direction. And in the lower-end, all they would need to do is produce several versions and iterations of their Rebel series at different price points. Canon has long done that with their Rebel film series (sometimes offering as many as 3 or 4 different versions of the Rebel film bodies, with only minor differences). It doesn't take much of an investment to crank these different versions out. It's primarily just packaging and marketing (in this case, packaging the same sensor and processor into different bodies at different price points). So I wouldn't be surprised if Canon introduces and aggressively markets multiple Rebel digital models, and it'll be just like the film days, where Rebels ruled the sales roost.
But one thing that these kinds of numbers don't measure is how well different brands are able to hold onto existing usership. And in that respect, Nikon is doing a lot worse than Canon is. Canon gets more Nikon defectors than Nikon gets Canon defectors. It's not uncommon to read about 10, 20 year users of Nikon equipment switching to Canon. And these are high-end users who spend a lot of money on equipment. For example, read the "despatches" of long-time Nikon landscape photographer David Noton.
http://www.davidnoton.com/despatches.php
In his August 2005 despatch, he writes, "I’ve gone for the Canon EOS 1Ds mkII (whatever marketing exec dreamt up that name?). With its 16.7 megapixel full-frame sensor it’s by general consensus established itself as the benchmark against which all other pro digital SLRs are measured, so I’m confident I’ve plumped for the best, for now. It's meant, however, junking my entire Nikon system, which has been a bitter pill to swallow."
His current gear is:
2 Canon EOS-1Ds mkII bodies
Canon 17-40mm f4 EF lens
Canon 24-70mm f2.8 EF lens
Canon 70-200mm f2.8 EF IS lens
Canon 24mm f3.5 tilt & shift lens
Canon 15mm f2.8 fisheye lens
Canon 100-400mm f4.5-5.6 EF lens
Canon 580 flash
And one battered Nikon F5 he keeps "as a momento" on his mantelpiece.
That's a whole lot of money that Nikon missed out on, but Canon got. And it's a bit more money than most entry-level buyers will spend.
These days, it seems like the 5D is the big switcher bate, with lots of Nikon users picking up a 5D for the advantages that FF offers, not to mention it's very attractive price for its quality of high res output. And I strongly suspect that the new 1D MKIII will draw even more switchers into the Canon fold. 1D MKIII unit volume may not add up to much when compared to the cheap lower-level bodies, but they contribute a significant dollar volume.