Atari was popular, and Sega was very popular, they both tryed to outdue Nintendo's handheld, and failed miserably.
I think sony will put up a fight, but they wont win. They seem to be making the same mistakes other companies made... (one being, trying to compete with Nintendo lol)
And even if Microsoft tryed to put out a handheld, I don't think they'd win either.
Also keep in mind that DS isn't truely a GameBoy... its kindof like a sidekick that is better than the superhero. XD
Nintendo stated that the SP and DS are 2 different systems and that SP is not obsolete yet. GameBoy should still have more advanced models in the future, and if DS does well, it may have some too. ^_~
On another note... there was an old system called the "Nintendo Game and Watch", long time ago, that had 2 screens. While it wasn't a system that you could play more than one game on... it looks EXTREAMLY familiar. LOL
Also, Nintendo just put out this artical on its press site!
Thanks the gods there will soon be 40% more DS in stores!
Check it out ^_^ I've attached it for you all. lol (its from dec 9th, so its new)