One other thing I firmly believe about Sony: every time they've entered a pro market, they have done extremely well - just look at video, video production and re-production, or their music studio equipment line. They may not have succeeded with consumer media carriers (Betamax, Memory Stick, blah) but when they want to be part of a market, they do a great job.
Sony has the marketing bucks, and the R&D depth to come out with spectacular products. They already make sensors for other companies, including Nikon. And the Konica Minolta people aren't exactly clueless either.
I expect Sony to be a very serious player if they choose to. Canon may need to worry, but as long as Nikon is relying on Sony sensors, they must worry, too.