Symptom
Applications that would normally use the F12 key no longer recognize it. Instead, the optical drive tray opens.
The F12 key cannot be reassigned for other use.
Solution
In the initial release of Mac OS X 10.1, the F12 ejects optical media or opens and closes the tray of an empty optical drive. While this provides functionality for keyboards that do not have a dedicated Eject key, this behavior can be an issue in some situations.
When working within an application, you can bypass the eject feature by holding the Option key while you press F12. Alternatively, you may use menu items where applicable in affected applications (instead of the F12 key). Some applications allow you to remap the function keys. Check the documentation that came with your application to see if it is possible to move the F12 function to another key or key combination.