It is a tall order, but Im all for privacy
What do you mean by 'unlock'?
Type a password on startup or into a screensaver?
Reset a password like the boot CD?
Control a physical lock of some sort?
Maybe you mean to lock/unlock--act as a dongle that has to be present for some app to run? That seems more in keeping with what an arduino would make sense for, although several of the others are possible.
Do you mean all for privacy, or all for piracy?? 
Do you have a locked-by-something-else Mac that you want to UNlock, or do you want an Arduino based system for LOCKING your Mac, so that only you, and only when you have your Arduino, can use it?
(If the latter, you are probably better off with any of a number of already invented wheels... sorry to be a Scrooge.)
Do you need to control access to the MAC, or just to certain files on it? Simple is good: Any decent Wordprocessor (e.g. Open Office's Writer) allows you to save a document with a password. The document can only be re-opened if the user knows the password.
Having said "go with already invented", if you just want the fun of playing with access control, the Arduino is a great place to go!!
(See....
.... for an example of access control. In this case the Arduino opens a door for people who know the DYNAMIC password!)
I mean to unlock the mac using the arduino on startup, in other words, to be able to unlock the actual software interface, not the hardware. Thanks for the tips, keep 'em coming 
I mean to unlock the mac using the arduino on startup,
Well in that case no.