The Pi hooks its UART pins to a login console at boot, and it should be straightforward with a suitable USB-Serial adaptor - I have the TTL-232R-3V3 from FTDI Chip - to view the results of the start-up process on a terminal window in Mac OS X.
The Pi has 26 general purpose IO ports on board, two of which can be used for UART (Universal Asynchronous Receivers/Transmitters) communications.
