Lornetw,
The stepper library already addresses your concerns about latency. See the
wiki for implementation details. As long as your app can push the commands into the queue faster than the machine moves, accurate motion should be possible. Ytai's X-Y plotter is a good demonstration of this.
A G-code interpreter should be pretty easy to make, if you limit it to the simplest command set. On the other hand, the optimizations and tweaks that need to be made to tune ANY controller to ANY actual machine usually (always) get complicated quite quickly. It might be attractive to be able to hack a $100 tablet and use it as both touch screen interface and machine controller (like I am doing now for something else) .... but to get all the bells and whistles of a package like linuxcnc ported over to Android would be a huge undertaking. Still ... if the 3d printer can operate on simple moves ... tantalizing thought.
Edit: Full disclosure -- I have a Tormach, I used to own a 'real' cnc, and I've done motion control with about half a dozen PLC brands (I build special machines). Ytai's stepper library actually looks like it is more capable than many PLC motion cards (a lot of them require a stop at the end of every move, so you can't program it to change speed in the middle of a path).
This looks interesting for access to cheap boards for interfacing to real-world circuits, maybe.