One word: DirectX
This wonderful API has given us PC gaming advances for years now, and the industry is nearly dependent on it. Should Microsoft make the Xbox 3 with DirectX 11 as a platform-exclusive, PC gaming would slowly go away. OpenGL is a good contender, but can you see all the development studios in a mass-migration?
Honestly, I think that this might be a good thing. If the next generation of consoles has full "personal computer" functionality, do we even NEED PC gaming anymore? The personal computer could make a move back to being an office tool, while game consoles become centralized entertainment hubs.
We're already seeing signs of a coming change. PC-accessory manufactures such as Logitech are making console accessories and input devices. They'll see no losses from a transition like this. Microsoft would see a dramatic increase in the Xbox brand's profitability, as PC gaming would move to the new platform with accessories such as lapboards and gyro mice bridging the control gap. This would leave Sony and Nintendo (maybe Sega if they feel like it) to dominate console-style gaming.
What's your opinion on this? Do you see it happening? Do you embrace it or oppose it?