I mostly agree with quarkx's response, though I'll point out a few things.
The PC platform came to life (as a gaming platform and Amiga replacement) when they started dedicating hardware to particular functions like graphics, and Microsoft made DirectX. The same goes for the games console when Sony entered the arena and pushed dedicated hardware. The Amiga succeeded for a while too because of dedicated hardware, whereas the game consoles of that era were really made from off-the-shelf hardware, and the PC's gaming abilities were a bit of a joke in comparison.
The PC doesn't consist of off-the-shelf hardware, it's all specifically designed for that role. Game developers have a pretty solid platform to build on, even though it is stagnating (allegedly because the hardware is too far ahead and the developers can't keep up - not sure - I can see how it might be true).
If the PC continues to make a retreat from high-end gaming, that will leave a niche (but which the console manufacturers will try to cater for - perhaps they are already, it's all a matter of taste), but requires a lot of attention for a new player to try and cater for it. If Microsoft were to screw up utterly again now (which they did with Vista, but I think they've saved themselves with Win7), then it leaves room for a potential new player in the desktop OS market, but that requires even more attention than the gaming market.
If a new player were to try and enter either of those example markets (that's assuming they have the backing of a big player like say a hardware manufacturer - nvidia is a possibility due to their lagging behind in general), it would be a MASSIVE achievement, perhaps dwarfed by the second coming of Christ, but I'm trying to find something to put it in its proper perspective
It would be almost as large an achievement if Canonical managed to push Ubuntu to be a major player in any consumer market, and they have quite a few important advantages going for them.
So I think an Amiga platform needs to target a niche market, and not a dwindling bunch of sceptical fans that liked something that Amiga was once.