Example of the MCP23017 using centipede library and sketch. KnightRider style. No Human interaction. The mus shield is wired up to my inputs i.e. Piezo sensors. thought not in use here the midi shield is used to output midi note data (again not used here). Stay tuned.
PS adafru.it has a library for the mcp23017 however Grumpy_mike uses an MCP23016 with just the wire library. Please note that in all cases the wire library in being used
The centipede shield was NOT used in this demo!
My i2c bus wiring 
in the above photo I'm using the mega 2560 but in my project and in the test I'm using the duemilanove. The only resistor used are on the 16 attached to the LED pins. Arduino pins 4 and 5 have no resistors at this point.
the library and the code can be found here centipede_shield [macetech documentation]