Use a small, light laptop such as a MacBook air.
Load up something like MAMP so you can develop dynamic apps with databases without an Internet connection.
In general, minimise trackpad use:
- Use an editor with good autocomplete (I use coda, nice as it autocompletes bleeding edge HTML and CSS, not so nice as it doesn’t autocomplete custom functions).
- Use an editor with built in live preview linked up to MAMP so you don’t have to switch apps much.
- If you’re using multiple apps, use them in different spaces and skip between them using the keyboard.
Have a 3G iPad or such device connected to the Internet to get at documentation.
Don’t try writing posts about how to develop in the back of a car on an iPad keyboard :(