Why do you think ATi sold out to AMD?
Not because business was that good, I can tell you.
If ATi had continued as an independent company, they probably would not have survived the HD2900 fiasco.
AMD poured a truckload of money into ATi. They could do that because the CPU market is much larger and more profitable, so it's not that difficult for a CPU company to sustain a GPU company. The other way around would be impossible though.
That's what all those other companies said

You know, 3DFX, S3, Trident, Matrox, Tseng Labs etc.
Most of these video card companies were pretty big at one time, but they made one mistake, and they were out of the race.
nVidia is not doing that badly yet though. While us guys on forums have been worrying about the fact that nVidia didn't have a good DX11 lineup, lots of people still bought their DX10/10.1 products.
And nVidia seems to be getting the situation under control now. The GTX460 is very promising, and there will be some lower end models next month, which will probably be equally promising.
I think nVidia has the advantage of being the 'Intel' of the GPU world. They're bigger than AMD, and their brand is much stronger. So they'll still have big OEM deals and lots of people buying nVidia just for the brand, even if their product lineup isn't that flashy for a while.
So far AMD seems to have just about caught up in terms of marketshare. But I think this is as far as it will go. GTX460 and its lower end cousins will probably claw back marketshare from now on.