It's tough to say this early. While Sony has proven it's dominance as the #1 console two generations in a row, it's a big question mark as if they can win again with the PS3. No company that has produced consoles has really came out on top more than two consecutive generations, and Sony could very well be the first console in history to pass that mark.
But, Microsoft has proven to be a threat to Sony's dominance, and if they can get their next console out the door before Sony's, then they might beat them out. But I could be wrong, look at the Dreamcast. Out a year before the PS2, but people would rather wait that one year than give the DC a shot.
Nintendo can do the same, but in this generation, Nintendo really doesn't seem to be in the same league as Sony and Microsoft. Sony and Microsoft want to be these "do-it-all" home entertainment machines that will play your DVD's, record your TV shows and so on. Nintendo is STILL sticking to games. I assume they will do so the next time around.
It will be interesting to see if people will fall for the extras the next genration as they did this one. It seems like the best selling consoles won due in to part having a DVD player built in. To me, that was gimmicky, and something like that doesn't matter today, as you can pick up a DVD player for $40. Then there's the whole online thing, which was also big this time around (of course how many people do you know actually go online with their consoles?). That too will not be as big of a selling factor. So the add-ons may not be as important the second time around.
I wonder how much Sony's next system will cost. With everything the want to cram in, on top of the huge amount of money they're spending on R&D, this could be their most expensive system yet. I can imagine Nintendo's next system being cost-effective (as they always have been), and Microsoft is looking to make their console cheaper the next time around as well. If 2 of the 3 next gen systems sell cheaper than Sony's, then they may lose the battle.
I'm also feeling that Sony is becoming arrogant with the PlayStation systems, much like Nintendo was with the NES and SNES (and GB). Nintendo was on top, they did things their own way, and carried that over to the 3-D generation (N64-PS1-Saturn era). Backfired in their face. Developers hated the expensive cartridge format, the market started to shirk, and eventually never was able to recover when the PS1 gained insane momentum.
Now Sony has had two highly succesful consoles, and there's a problem with both. Both consoles offered as little as possible for the most money. What I mean is, they offer the least amount of power and capabillities, and yet can ask top dollar for it. Just look at the market now. Nintendo's Gamecube system in the middle in terms of performance, and it's the cheapest, retailing for $99.99. The Xbox is the most powerful this generation, and it's going for $149.99. The PS2 is the weakest in terms of performance, and they still ask for $179.99. How is that? Will the next PS3 be the same way, and will people tolerate it? Then there's the whole development thing. I've heard that the PS2 is the most difficult to develop for, will that be the same with the PS3?
But it's a waiting game. Sony could very well be #1 the next time around (and they better damn well have a system good enough to live up to that claim). Unless a competitor has a good enough launch before the PS3, then Sony may be able to take this. Right now they're riding high, but it's also their game to lose if they slip up.