If your Mac is set up to require a password when being awoke from sleep or the screen saver is it possible to log in as a different user? If so how?
If you enable "Show fast user switching menu" in System Preferences → Accounts, then the lock screen gains a new button "Switch User..." which will take you to the login screen.
On Mac OS X Snow Leopard and previous, the Switch User button appears on the password dialog:
On Mac OS X Lion, the button appears at the bottom of the screen: