I've borrowed from Peter Bukowinski's script that will lookup when an AD user's password is due to expire, and expanded upon it to run as a task in the OS X menu bar. The applet includes preference settings for the following:
- Adjust how often you are required to change your password (5 – 365 days).
- Automatically launch the applet at logon.
- Display the date your password will expire on the logon window (requires admin permissions).
This applet is open source and hosted on Google Code: Password Monitor.