Apple is the best contender to bring one to market successfully however Apple's software approval process will kill it.
Next in line Sony and Nintendo.
Sony is known for overpriced great hardware, shitty support and on top of that until they lowered their prices they pissed off a lot of developers.
Nintendo on the other hand upgrades its platform every few years and does a great job of making software backwards compatible, unfortunately unlike Sony (Ericson) Nintendo has never made a Phone so their entry into that market will be difficult because they are behind the curve for Cell Phone R and D by about 20 years.
Now if Apple was willing to co-brand (which they wont) and bring Nintendo's reputation (and loyal customers) into the iPayTooMuch Platform they could have a solid winner that would rival the PSP. I loved my PSP but it was a bulky and closed so hacking it was a P.I.T.A. (attempt->brick->repair->attempt->brick->repair->attempt->brick->drink vodka , turn on xbox and say screw it). Same reason I gave up on iPhone, they locked their phone with AT&T I had T-Mobile. Hack-update-Hack-Update it was an annoying cycle plus apple lost out once I hacked my iPhone I was able to put free software on there which eliminated the requirement to PAY for anything from itunes before deciding if I wanted it. While Apple claims that its policies are to enforce brand management etc etc They basically don't want to admit they fucked up by being exclusive with AT&T when they knew the phone was not unhackable (only a sucker would believe something like that).
The first company to successfully bring a true all in one portable device that doesn't lag, actually works and has a compliment of reputable developers actively supporting that platform will win. I know that statement is kind of obvious but if thats true shouldn't someone have achieved this already?
Oh and if no one remembers reading their slashdot and techdirt Nokia pulled the plug on N-Gage servers a few months ago so those that made the investment in software and hardware were effectively screwed. I feel the same way about Nokia as I do about Sega or Canon or my little brother Great Ideas, No follow through. For example the N series phones great hardware No support from Nokia and No support from developers = cool device but dead platform (Anyone remembering the Sega 32X Saturn Dreamcast or Canon Bubble Jet Series Printer right about now?) The unfortunate reality is as much as I HATE microshaft if I wanna game I have to do it on an Xbox 360.