There's a whole bunch of loose ideas running around, here's some more:
Hong's "MMO that people could make sub-servers" (paraphrased) was called NeverWinter Nights. While a cool idea, did it really take off? Did it replace table-top gaming?
No, and here's why: The technology requirements were too steep for a majority to use. That's not saying y'all didn't play it. The problem was getting everyone in your gaming group to have the right technology to play the game. Everyone had to have a newer PC with a GOOD video card. Everyone had to have broad-band internet. Everyone actually had to know how to connect to a network game session.
There's plenty here (most of us on an internet forum are tech saavy) that can get our home rig ready. Try getting your entire gaming group ready. That's like being the IT guy for your gaming group. If your entire gaming group was as tech saavy, they'd be on this forum. And given how much people complain about their gaming group, it's obvious that they don't think their gaming group is online.
From all the "virtual tabletop" apps I've looked at, they all look cumbersome enough that half of my gaming group would get confused and frustrated just trying to connect to the game. One of my friends is running a Win98 box with dial-up in the woods, another a Mac. Another is a Luddite high-school CompSci teacher (seriously). Half of them of in another state (trying helping them figure out how to install Java Runtime on a Mac, so they can run the same app).
Part of the problem is that there's some many flavors of the technology, and it's not a priority to everyone. So getting your entire group onboard, so you can use the same app and get "networking" synergy is very hard to do.
Whereas, running just your character on a device, with no connection to other players is the easiest to get working. And even there, the applications fail. I have yet to find an app that makes it easy to play D&D. I've seen plenty of apps for managing your PC between games, or for creating the adventure. But they are all too cumbersome for use during the game.
For folks harping on the iPhone, just get an iPod Touch. It's the same thing, without the phone and related baggage...

On the Kindle, it's an eBook. That technology is still changing. It will get better. In time, a product that looks like a cross between a Kindle/eReader and an iPod Touch will emerge. That'll solve that problem of small, but not too small, readable, yet useful.
Personally, here's what I found to work, to some extent:
d20srd.org. One of the best sites for looking up game rules/stats. Works good on a laptop at the game table
MS Word: I write my adventures in it, and run them straight off the screen. I include hyperlinks to d20srd.org for things I'll need. It's not perfect, but I haven't found an app I like for managing my adventures AND running them
MS Excel: I put my PC into my hand-built spreadsheet. It has just the fields I need, where I need them (I use tabs for different aspects like inventory, stats, skills, combat, spells). An app to do JUST this, with quick access to the screens, simple UI for healing/damaging, and simple controls for adding/removing equipment (shopping/using supplies) would be awesome.
mIRC: a long time ago, a part of my group played online. We used IRC chat. The DM was the least tech saavy, we sent him to a web-chat site (hosted by my ISP), and the rest of us used mIRC with some handy scripts to manage our hitpoints and weapon stats (AC/THAC0). The reason that it worked is that our scripts ONLY managed hitpoints and stats. It wasn't trying to abjudicate the entire game. It handled the basic "heal Janx 5" and "Janx attack sword" to tell the GM how many HP I had left, and what I rolled with my weapon bonuses.
In none of these examples, were a dice roller needed. The only time automated dice rolling is useful, is when it's integrated with the parts that "know and need" the bonuses and die results. Just rolling virtual dice is useless.
Can an iPhone app do all that? I'd hate to GM from such a tiny screen, I need more info than that. It might be OK for PC management, if there was a good app for it (that didn't get sued out of business by WotC). It'd be a pain for typing text (that keyboard is small). Buttons could be made, however...
I don't see an iPhone app coming out. WotC keeps a choke hold on software, even in the 3.x days, that makes it cumbersome (you have to display the rules, etc).