Hi...
Just for the record I've been looking into how to get the Arduino to interface with the MCP23S08 "8-Bit I/O Expander with SPI Interface" IC from Microchip.
I have successfully used it for both input and output but haven't tried the interrupt functionality. The code's very rough and details are very much in note form but it might be helpful for others wanting to use the device or get other SPI devices working with the Arduino.
My notes are here:
http://code.rancidbacon.com/LearningAboutArduinoMCP23S08
A demo video with gratuitous LED use can be found here:
--Phil.
Edit: Okay, so the embedded Flash link didn't seem to work... 