First of all, let's NOT make this into a Canon/Nikon discussion. Start another thread (or join one of the many) if you want to. I want to discuss Sony's business strategy.
So it seems to me that Sony has a fairly dominant market position. Aside from Canon, Sony is making the imaging sensors for Nikon, Pentax/Samsung, and of course themselves. In terms of sensors, it's a four-way race between Canon, Sony, Fujifilm, and Kodak (who makes the Oly sensors I believe). Fuji
With Kodak/Oly/Panasonic/Fuji being fairly minor players, the way I see it, Canon is in direct compeition with Sony. Pentax and Nikon are big players, but both are completely at the whim of Sony delivering them a sensor.
In the consumer market, I think the likes of the K100D, Rebel XTi on the low-end and D80/K10D on the high-end has it covered. They already deliver pretty good image quality, and for the majority of consumers (not us on this forum, mind you), the selling point is big features, not pixel-by-pixel image quality.
The biggest sector that Canon had monopolized was the pro-sector. Full-frame has swayed a lot of people, but bigger than that, Canon has had zero competition in the sports arena for years. The D2h had long fallen behind in both MP and high-ISO, and the closest thing since then, the D200, was nice but was nowhere near the same level of autofocus and still lagged a bit with only 5fps.
For a long time Canon was the only company with the technology (and resources) to develop full-frame sensors and high continuous shooting sensors (coupled with high MP). Then Sony announced a 12MP, 10fps sensor - finally something that could actually compete or even top the 1d sensors. And with the D3, they've shown that they can develop full-frame sensors as well. Both products - full-frame sensor, and high framerate, high MP crop sensor, were finally legitimate alternatives to the Canon full-frame and Canon sports sensors.
So this begs the question: Why did Sony not seize the opportunity to put out their OWN full-frame camera and their OWN high-fps sports camera? There was a huge potential market for legions of Pentax users, Oly users, and even Nikon users ready to make the step up to either a full-frame or truly pro body (with AF and high FPS), but resisted the Canon system, and with both or even one of these bodies, Sony would have instantly catapulted themselves into #2 in the pro market, all but essentially killing Nikon in the pro sector.
Instead, by giving this sensor to Nikon, I think they've essentially killed any chance that their system will gain any traction, especially in the pro market. Since Canon was the only game in town, Sony could have easily penetrated into the sector, especially with the Nikon prices demonstrating how low they could have sold cameras for. Now if they choose to enter the pro segment, they've not only got to compete with the $4000-$8000 Canon bodies, which was doable, but they've got to compete with an $1800 Nikon body. I can't see many pro shooters and any major photo agencies making a switch to Sony with such a low-priced Nikon and an installed base of Nikon or Canon lenses.
What's the reasoning behind Sony's strategy? They didn't want to ruin their business relationship with Nikon? Understandable, but to me the Sony relationship with DSLR manufacturers is one-way: Sony provides the sensor, and Nikon/Pentax/Samsung simply buy them. They're not giving Sony back AF secrets, or body design secrets, or anything like that - Sony already has that from Konica Minolta, and their own development.
They want to proliferate their sensors into everyone's cameras? This has worked well for Sony in the consumer market - Sony makes more from the combined Sony, Canon, Pentax, Casio, et. al. sales than they would have from Sony sales alone. But the difference is that the consumer market is undemanding of image quality - Casio can go ahead and stick some Kodak sensor into their camera, with 2x worse noise performance, but most consumers will still buy their cameras.
This isn't the case for the DSLR market, and ESPECIALLY the pro DSLR market. As Canon has demonstrated over the past few years, if you have bar-none the best product, you can wipe competitors out. If Sony kept the sensor for their own D3 and D300, and left Nikon high and dry with current D200 and D2h technology, there is zero chance Nikon could ever compete, and it's not as if Nikon could turn to some other sensor manufacturer like Kodak - they can't make a 10fps sensor, and can't make a sensor which comes close to the same image quality.