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?
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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:
The way FileVault 2 works is that the OS sets up a recovery partition that is used to store the encrypted keys used to unlock the encryption. The recovery partition is created for all Lion installations and is used for maintenance of the system, but will be a requirement to have if you wish to enable full disk encryption on the boot drive.
When the system boots, it accesses the recovery drive and loads the login screen to present to you. When you then supply your password it unlocks the boot drive and continues to load the OS and your user account before dropping you to your desktop. As a result of this, the preboot login screen may show much quicker on systems with FileVault 2 enabled than on those that do not.
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.