Of the three companies at it now, Sony is currently the most likely to exit the market, and probably not for a generation or two. Nintendo's profits from consoles are too great and their strategies too effective for them to exit... well, ever, really. Microsoft was stubborn enough to release the 360 even though the XBOX lost them money, so they're going to stick to it at least until they make an overall profit (or take a huge loss). Sony, on the other hand, is at the other end of that: they've made a big profit and are now seeing heavy losses (heavier than ever before, even). Should the PS3 continue to be largely unprofitable, their stockholders will most likely demand an exeunt from the market within a generation of consoles.
As for who might enter the industry, anybody with a fair amount of money begging to be invested in video game systems is fair game at this point. Nobody thought Microsoft would enter, being the giants of PC software (and not especially noted for games), but they did. There actually is a strike against Apple entering the industry that very few people know about any more: the Apple Pippin. It did so poorly that it stands as the world's worst-selling video game system ever; most people haven't even heard of it. People at large may not remember it, but I doubt Apple's forgotten the sting of defeat it carried for them entirely.
Sky Render - Sanity is for the weak.
