I'd say Sony is a very serious contender. Canon and even more so Nikon, who doesn't make its own sensors, should be taking Sony seriously as well. In terms of Professional use there is going to be some inertia in uptake as anyone with a large investment in lenses and peripherals is going to be slow in changing directions.

The full frame could be a shock to the market if it comes out in the price range of a 5D.