Not a very usable design. I know that the mockup tries to fit in everything that's been recommended so far, but it could do with some simplifying. Here are some thoughts I've got after looking at it a few times:
- remove the touchpad. For the size of device(s), you want larger buttons for typing (this is the reason buttons were used instead of button pad on N900 - expectation was that of coding not navigation); replace with 5 buttons (up, down, left, right, center/select - last button might get 2nd function as additional Fn button or maybe make that 2nd function the Multitask button)
- no more than 3 stated functions per button: main = large and white, 2nd = top right superscript and also white, 3rd = top left superscript and red or blue
- ditch the multitask button's size. It should be as large as a number button if on that row
- move Fn button to under the Enter button
- span the size of the Enter button to be 1 row
- move Alt and Sym buttons to the left of the spacebar
- move comma and period buttons to the right of the spacebar
- tilda (~) button could go next to the "1" button, fitting a more common keyboard layout

I think that's all for now. Thanks for taking the time to make a mockup. It gives a bit better of a place to talk about what's wanted, what's usable, and what's doable.