A suggestion for Hardware and Software.
Add an I2C connecteur on the board ( 2 wires ) and add a library to read/write I2C peripherical
like eeprom / Real Time Clock or Devantech Compass...
There seems to have been discussion of I2C on the arduino before, including a link here:
A library in the wiring language for communicating over I2C, mind I'm a newbie with the arduino so I don't know what it would take to make this work. I2C certainly seems like something that shouldn't need a separate port, my recollection is that it's designed to be easily interfaced with.
the Wire library is included in 0006 release. check out the news forum for details.