The amount of effort and expenditure it takes to miniaturize and package electronics into a rugged, phone-sized gadget is completely out of reach for hobbyists. Openmoko came close but they had real funding.

Yes it is possible to put a beagleboard, battery pack, etc. into a much larger box like a pelican case, basically the size of a netbook or not much smaller. It would even be pretty cool in some ways. But it wouldn't be a pocket computer.