There are many differences between touch typing with all your fingers on a keyboard, and just using your thumbs on an iPhone.
If I were to design such a keyboard, I would adopt some of the ideas from the IBM's SHARK layout. The hex shape is an interesting idea, although it might not be suitable for everyone. Now the product based on that technology is called ShapeWriter.
But instead of having one home position in the center where the space bar is in the SHARK layout, there would be two: one in the center-left, and another in the center-right containing the E and the Space-bar, and the other keys around them ordered in frequency.
See also the Microsoft Origami dial keys.

Another way to touch type would be one-handed 4-finger touch typing, in which it would be more similar to Maltron right-handed keyboard.

These are interesting ideas.  I think they have more appeal if you plan to do a lot of writing on your mobile device in a very noisy environment.  Personally, I would not want to.  Rather than working on developing new skills at either finger tapping patterns or memorizing squiggles with a stylus or a finger as words, I would much rather have something that simply reinforces my memory for touch typing with two hands.   Have a Colemak layout on my iPhone would simply reinforce the pattern I have chosen to use when writing.  If I really found the need to do a lot of writing on the go, I would much rather have a bluetooth wireless foldable keyboard (or wifi) to touch type on. 

Having learned things like Grafitti and Fitally in the past, I am more interested in technology that doesn't ask me to take up brain real estate with what I would rather have be occupied by piano patterns. 

I am just skeptical.  With limited real estate, speech at 200 wpm with a smart way for correction makes a lot more sense.   An iPhone with MacSpeech Dictate setup with a Jawbone bluetooth headset would be the way to go. Now that
would be awesome!

Samurai: I'm thinking of what people would do in a situation where you don't have a surface large enough to touch-type on. For the situations where you do, I'm sure you could do well with a virtual keyboard. Maybe a laser-drawn one like the one I described in my keyboard modding post. I'm sure they could make such a board for mobile devices eventually.

But I see a lot of people typing short messages while standing on a bus or even walking in the street! In this day and age I think that much of the mobile typing needs to be truly mobile in the sense that you won't have a space to put down and fold out your typing device. Thus it will have to be very compact and usable while holding it in one hand, like a mobile phone. And it should still be a lot better to use than today's mobile phones.

I'm also thinking about how interesting it is that it seems so easy to use that left-handed Colemak layout that simply mirrors the layout when you press the modifier key (Space). I think ds26gte is right: It does help to be on a layout that you know well, even if you're using it differently. So maybe my idea could be combined with that thought?

Maybe if the thumb had one keyboard row on the pad. One row is 12 keys long which may be overmuch, so possibly just the main 10 (ARSTDHNEIO and corresponding keys on the other 3 rows) and then an extra row for the less used stuff. Then the other fingers would control which row you'd type on - for instance, no other finger buttons pressed would give the home row while the index finger gave the upper row etc. The touchpad "row" wouldn't have to be linear; to make it reachable for the thumb it could be curved a lot and still be recognizable. To get shifted characters there might be a sticky shift button (thumb or other finger - not sure). Now you could have a Colemak layout combined with my idea!

Envisioning is a great sport.  ;)

latest - switched left and right hand side to go over two rows

I think three-finger typing would be faster than two-thumb, i.e. hold it with one hand an use index-middle-ring on other hand to type. Use it like a 10-key. I have only ever typed on an iPhone once, so I'm not sure, but this sounds fast.