How can I see the system status during boot? I would like to monitor the messages generated when I boot my Mac, similar to what you on a *nix box. Is there key-combo to press/hold that will let me do this?
You have to press cmd + v upon startup to see the boot output. Press this right after the chime. 

If you want to make that permanent, you should probably do something along the lines of: 

sudo nvram boot-args="-v"


To undo the above:

sudo nvram boot-args=


in the Terminal. (/Applications/Utilities/Terminal.app)

note: I haven’t tested the above commands, but I’m sure a google query will help you there if you want to make it permanent.