Forbes.com has an interesting article titled, "Why Apple Could Kill the Nintendo DS"
While there is not much that could kill the DS other than, well, the next DS, the article does argue that Apple's iPhone may soon become a successful player in the handheld gaming market.
The first point is that Steve Jobs is set to announce the App Store which allows developers to create software for the iPhone (rather than just Apple itself). This will allow users to download games and could create a thriving developer community.
Secondly, and I think more importantly, EA will release it's next huge "Sims"-like game, "Spore," on the iPhone the same day it goes on sale for all other systems, including the DS. If it is even mildly successful, other large gaming companies are sure to take notice and will begin to develop and release AAA games to the iPhone.
Thirdly, the aging DS is currently at a technological disadvantage. The iPhone is like alien technology, discovered by the defense department when monitoring a worm hole (did anybody else watch The Andromeda Strain?), while the DS is like the tried and true Space Shuttle. It's not that exciting, but it gets you to the space station every time. That being said, a game like "Spore" will, arguably, look and play better on the iPhone. However, Nintendo won't sit back and is sure to update the DS soon.
Of course, the one huge factor in all of this is the price. The DS is $129. The iPhone is $400. The DS has hundreds of games. The iPhone has about as many games as I have fingers. Nintendo is a gaming company. Apple is not.
Personally, I'm all for Apple getting into gaming. The more players the better. It's not like Nintendo is going away or will start to make crappy games if Apple is all of a sudden successful in gaming. It'll just make them all the more competitive.