The quickest way to have working tablet would be to adopt existing design and focus on software.

We can wait for such hardware to become available, or try to re-negotiate with some other vendors to provide us with hardware, while at the other side we provide the software.

The hard way would be to try the following approach:
tablet = casing + mainboard + battery + charger + touchscreen + additional extras (gps, gsm, accelerometer, etc...)

Based on some projects I was involved with, case prototyping is expensive, time consuming, and tedious work. Due to the initial costs, it would be advisable to produce such shell that can be re-used/upgraded in future (e.g. enough space to upgrade touchscreen, battery, mainboard, in future).

Designing electronic is non-trivial thing as well. As seen with OpenMoko Neo Freerunner and beagleboard, it takes several iterations until all issues are resolved. Perhaps we should try to cooperate with some other projects (Freerunner gta04 e.g.), so we can reuse their hardware and insert it in our shell.

Software, there is not too say much about it, except that we need a platform that will allow future upgrades.

There are couple of questions that we need to answer:

1. How do we fund such approach?
2. Can we find a sponsor who will be interested to finance the design?
3. Do we have required knowledge?
4. How much would cost the final device?
5. Can we finish it?
6. How much time does it take?
7. Can we perform above tasks in parallel?

Neo Freerunner was a project that was ahead of its time. I still think it has better casing than many commercial phones.