Instant.. ...deal-breaking... ...showstopper.
And don't even start with Android.
A real cpu deserves a real distro - one with real repositories,
not just some blow-hard f@rt-app prop-store.

if you can cool it, and if you can power it - good luck with that
I was considering along these lines last month but just an i3
with the integrated graphics would be adequate for portability's sake.
Any GPU you tack on will chug power and generate enough heat
(along with the CPU) to tack on an espresso maker.
(I still have visions of steamed mocha venting out from a cpu nozzle)

Mandatory minimum

7" is more practical unless you want to lose market share.
10inch will consume about twice as much power = batteries.
7inch, while not pocketable, is a lot more stowable.
I just bought a Samsung Tab for a pilot where the formfactor
was a primary consideration.
10inch would be too big for the killer-app uses of a real cpu in-hand.
Besides - you spec an HDMI (I would suggest DSUBvga as these are still
ubiquitous, even for the next 2-3 years) for the occasional big-screen experience.

All these things are not particularly easy to integrate,
Don't forget USB3
Lose the FM radio - it is just extra expense.
Lose the DVBT - it is just extra expense.
Accelerometer <Vibration> if you use invensense, could be part of the gyro module.
WiMax has issues - stick with wifi -
3g you can do with gammu and Huawei stock modems.
Expect GPS and compass on the same module.

What I had in mind is a bit more like this:
in a smaller case and without extra junk like dvd/blu-ray
GIGABYTE H55N-USB3 is my current favorite for this type of build.
I have built mATX systems for years (I still have my first low-profile aircraft aluminum case
which now holds an EG45M-DS2H and a lot of empty space due to ssd and no other drives).
The new mini-itx motherboard begins to make diy slates possible.
Meego, if they get it right, would absolutely scream on such a machine.
Power would need to be something like this, as 12v batteries are anywhere.

All this is mere whimsy if Nokia delivers an 'N9' running on a Meego
that does not have a whole raft of proprietary closed-source issues.
And does not immediately toss the whole Meego idea under the train,
like they have done so often in the recent past.

(or almost whimsy - i am actually building such a beast,
but it is for replacing my current work machine which is getting abit dated,
and will function as my version of a laptop.)