I don't see an immediate competitor for Nintendo, at least in the handheld market, for years to come.
Iphone/Ipad be be somewhat of a competitor, but I see it more as an indirect competitor, as Iphone is not primarily a game machine like 3DS is. It would be like calling the PC a direct competitor to gaming consoles. It's just silly. Not to mention, Iphone seems to have far more mini game type of software, whereas DS and 3DS games seemed to be more serious core games. I think Iphone and 3DS should be able to coexist and thrive equally, since they serve entirely different purposes and cater to different audiences - they just also both happen to play games.
Sony, from my perspective is dead in the water at this point (talking in terms of handhelds). They have already waited too long to release the PSP2, and at this point it looks highly unlikely they can rush it out there any earlier than 3DS. I'd say best case scenario for them is a near similtaneous launch with 3DS, but even then, most 3rd parties seem to have already began 3DS so it would probably get better support. 3DS coming out first (probably) is important because they will have a greater install base off the bat. If PSP2 DOES come out soon, my guess is it will not have 3D, as Sony probably weren't aware of Nintendo having a 3D capable handheld before March. However, if Sony waits it out longer to follow Nintendo and implement the 3D, they could take away some of the edge from Nintendo, but they will risk falling behind for releasing their handheld too late. So Sony seems to be in a tough spot.
If you also look at the software on 3DS, MGS, RE, Final Fantasy, Assassin's Creed, Saint's Row, Kingdom Hearts, Dragon Quest, etc.. this is all stuff you'd expect on the next PSP, but instead it's on Nintendo's handheld. So they could potentially bring in many Sony fans. With these titles, Sony will have a hard time coming up with reasons to keep their existing fanbase on their handheld, let alone bring in NEW consumers.
So, no, I don't think any company can really reach Nintendo in the handheld sector for quite a long time. I think if any company has a decent shot, in terms of being a DIRECT competitor to Nintendo, it would be Sega. They are one of the few companies left that still pump out a decent amount of quality software on a consistant basis. But they'd have to market it well, make it cheap and accessible (unlike Gamegear), and have great 3rd party support.
Obviously that won't happen though, but that'd be sweet if it did :P
