Answer
This question was answered on July 6, 2007.
Much of the information contained herein may have changed since posting.
To reset a Mac OS X account password, startup the computer using an Apple Software Restore disc (included with newer Macs) or a Mac OS X Install disc. It is best to use the same disc from which OS X was installed. Do not use a disc with a version of OS X other than the one installed on the computer or it may not work. For example, if you reset a password on a computer running Mac OS X 10.3 with an OS X 10.4 disc, you will not be able to log into the account.
Insert the disc, restart the computer, and hold the C key when you hear the startup sound. You can release the key when the gray Apple appears.
It is normal for the computer to take longer to startup from a CD or DVD. The disc will automatically start the OS X installer but you do not need to reinstall OS X. If you are prompted to select a language and you do not see the menu bar at the top of the screen, click the right arrow button to proceed to the next step.
The method of resetting passwords varies slightly depending on the version of OS X on the installation disc.
Mac OS X 10.2.x through 10.3.x:
Select Reset Password from the Installer menu.
Mac OS X 10.4.x:
Choose Reset Password from the Utilities menu.
Utilities menu
* Select the hard drive (e.g. Macintosh HD) at the top of the window.
* Select the user account you wish to reset. Important: Do not select "System Administrator(root). This is a reference to the root user. Do not confuse it with a normal administrator account.
* Enter the new password in the two boxes. In Mac OS X 10.4, you can also include a password hint.
* Click the Save button.
* Click OK on the message confirming the password has been reset.