As a more direct reply to this topic, I'd say that the Apple platform is an equally significant risk to Nintendo and Sony, and possibly the greatest risk to Microsoft.
Apple and Nintendo bump heads because they are 1) both significantly concerned with the portable space and 2) both significantly concerned with the casual space. Apple and Sony bump heads because they are both concerned with the convergence device space, which puts Sony in the akward position of choosing a competitor with the PSP2: either make a gaming device and compete against Nintendo, or a convergence device and compete against Apple. I envy them neither choice. If they make a PSP2 at all, they'll likely try to split right down the middle, because going up directly against either one does not sound appealing.
Microsoft's threat is totally different. Their threat isn't against the 360, it's against the Netbook and Laptop market, where they are the dominant player in OS. While it isn't as formidable a threat in this regard, it's also not striking at peripheral markets like Sony and the PSP, it's striking directly at Microsoft's core profitable enterprise.
I can genuinely see the iPhone/iPad hurting any and all three of these companies as it carves out its own space. All three will need to position themselves well, because Apple isn't a slouch.