With FileVault 2, you can encrypt the hard-drive such that users on the computer can decrypt it with either of their passwords
How does this work?
My guess would be that it really encrypts the drive with a master key, then encrypts copies of the master key using valid user's passwords. Enter your password, and the system decrypts the master key. It then uses the master key to decrypt the data.
I found a cnet article which says:
This doesn't exactly tell (and the exact details are probably not out there), but it could be as I originally said. What I described would be more secure than just storing the master key in plaintext.