That could work, and give the phone some nice extra gripping surface when being held.

However, wouldn't that introduce quite a bit of additional parts (holders + springs/clips to lock batteries in place) that wouldn't be as rugged as a simple back cover with a thick kickstand?

BTW, Estel, adding a bit of space to the plain cover for mods (touchstone, full SD cards?) sounds like a good idea. You can use some small protruding things about the thickness of 1 and a half SD card to make additional space and still hold the battery in place.

Side note, if we could ever find a way of interfacing other displays with N900 display controller, we could probably make a 4" or more N900 out of the current board.

The keyboard can probably be done like the Game Gripper - have plastic keys protruding to press the keys on the board, or make a separate board for the keyboard, with FN keys, etc. hooked up to (I2C? internal USB hub for extensions such as gyroscope etc?)