For display, you can hook up video display glasses to an iPhone today, then use ScreenSplittr to mirror the display to the glasses. With some Windows Mobile phones, like the Touch Pro or Touch Pro 2, you get TV out "out of the box" with no extra software (do need a special cable). That gives you a 640x480 screen for ~$300 with the myvu crystal glasses (myvu.com). Does anyone know of an Android phone that supports this? There were reports that the G1 has the appropriate hardware but I haven't seen anyone get the tv out to work.
The sticking point is the rest of the interface. There's the BlueInput drivers for Windows Mobile, that lets you hook up a bluetooth mouse and keyboard. You could then use RDP to connect to a machine elsewhere and have it host your IDE and compiler. Almost certainly not the "right" way to do it but it's a start.
Multi-touch interfaces turn out to be hard with video glasses, at least on current phones -- you don't see where your finger is. Of course you could ignore the video glasses and just look at the phone screen, but then you have a small screen.
May be worth thinking through what kinds of development are best suited for a person walking around. Are you planning to camp out at a coffee shop and work for six hours straight, are you planning to make quick changes to an existing app "on the fly" as you think about them, or something else?