The current plan is to build an interface for every hardware device the Arduino micro controllers are going to need to interface with.
A list of devices and current progress with each device can be found under the wiki page of this Github repository.
Most code, for interfacing with hardware should be placed in a Library, see this tutorial for information about writing a library for Arduino.
Please check out the GitHub hosted wiki on this respository if there is any information you think that everyone else should know about.