@Dodece:
Competition is indeed a good thing, but history has shown time and again that the best-selling games on any of Nintendo's consoles are Nintendo games. Even the NES was this way, and it still had massive third-party support by the end of its lifespan. The first year or two in particular of any given Nintendo console tends to have a great deal of top-selling Nintendo titles, and a comparatively meager third-party showing. The Wii is shaping up this way too.
The way Nintendo markets vs. Sony or MS is quite different. Sony and MS rely all but entirely on third-party titles to draw in revenue, with their own titles usually making up less than 10% of their bestseller game library. Ergo, if third party dries up for Sony or MS, they're in big trouble. We've seen a fair bit of that affect the PS3 this year, and the 360 had some of that happening at the very start of its launch year. Nintendo's strategy, as ever, is conservative: they have several studios producing top-quality titles for their consoles at all times, assuring that they won't be up a creek if third-party support is lacking. It has the negative side-effect of making third-party developers have to work harder to get a profit off of titles on a Nintendo system (particularly in the first year or two), but the benefit to Nintendo is far greater. In terms of looking out for number one, Nintendo's plan is unbeatable. Not to say that MS and Sony's strategies are necessarily worse, but they do have a lot more risk attached.
On a semi-related note, I suspect that the Achievements system has helped boost some 360 games' sales. There will always be people who are addicted to showing off how "good" they are, and the prospect of upping their gamer ranking will look appealing enough to encourage them to go buy a few games that they'd never get otherwise. Granted, it's likely a fairly small part of the 360 market that does this, but it cannot be denied that this has an impact on third-party sales (largely a positive one, at that). Not surprising that Sony's developing a similar system to be integrated in the PS3 Home software.