There are quite a few smartphones and PDA's which are programmable. I own an old (circa 2000) Palm Pilot and have a C compiler and a Lisp(Scheme) interpreter there. (Also there exists Forth compiler). Nokia 770/N800/N810/N900 series is as powerful as the development platform as your Linux machine. There exist on-board development environments for Windows CE/Pocket PC/Windows Mobile (even Common Lisp and Emacs) and Symbian (including Python).
So in this respect the iPhone/Android platforms are a step back from truly open mobile development environments. This step back is the result of the backwardness of operator-controlled American mobile market.
The main problem of the onboard development is the lack of convenient input. So I think the main task is to improve projector technology and ergonomics of projection keyboards (use more natural hand movements?).