@ Jaseone - anything. Not even just for iphone/mac coding. References, (class documentation) compilers, though that would be limiting, debuggers, IB's, etc..
@ Stratification - I used to dev for palm PDAs, and I did it all onboard my own palm. It was kind of slow, but I find coding on the go to be very useful. Sure I could just write in notes, but then I can't test my code or anything.
I think it'd be possible, though hard, to get a full debugger, code editor, and even IB onto the touch. Then just provide a way to get your stuff on and off your iPhone.