Philip Ryan thinks Canon and Nikon should be worried:
Sony has, once again, radically changed the world of DSLRs with its A77. Plus, it has done so without compromises in the shooting experience or in image quality. The A77 now reigns supreme over APS-C format DSLRs. Neither Canon's EOS 7D nor Nikon's D300s can match the A77's AF speed or resolution.
Seasoned photographers with extensive lens collections will likely have a hard time deciding if they should switch to Sony...
I've not tried Sony's new cameras, so perhaps I am missing out on some exquisitely sublime new features. Or whatever. But arguments like Ryan's overestimate the significance of camera bodies towards the overall photographic experience.
Camera technology advances quickly. The major manufacturers release improved models every few months, with the depreciative effect that new bodies start loosing value as you're driving home from the store. In two years, a $2000 camera may be considered obsolete, even though it works to capture perfectly salable images.
In contrast, lenses- you remember, those bits of glass on the front of the camera that actually form the image- stay current for years. I still use the same lenses I had when I bought my first dSLR. They took great pictures then, and they take great pictures now. Some of them, including my MP-E macro lens, are worth more than when I bought them.
Different companies manufacture a stable of compatible glass for a range of applications: telephoto lenses for sports and wildlife, portrait lenses for weddings, tilt-shift for architecture, and so on. When you buying a camera, you are buying into a lens system that will work from now until the expiration of the laws of physics. Lenses are an investment. The camera back, on the other hand, is an ephemeral throwaway.
It's a fact of the SLR market that some years, Canon will make the best camera. Other years, the honors will go to Nikon. Or, in this case, Sony. Playing the who-has-the-best-camera-now game is simply not winnable for anyone but the manufacturers. If you are buying an SLR for the first time, you'll still want to evaluate which lens system is best suited for your genre of photography.