New Feature request: Secondary Field

There should be a module as part of CCK called "Secondary Fields". When turned on, each field has a checkmark beside it (on the "Manage Fields" page) to add a "Secondary Field".

The way it works is if a certain value/criteria is met in the main field, the 'Secondary Field' will activate. The Secondary Field works just like a regular field, except it only activates (with Javascript) when the criteria is met (at the time of node creation).

Example. I have a basic article node that has a Select field for "Special Announcement". The options are "Birthday, Wedding, Anniversary".

If the user selects "Birthday", a 'Secondary Field' appears where they can now specify who's birthday it is (from a nodereference field perhaps).

Anyway, this feature would come in really useful on a site I'm working on - so I thought I would go ahead and request it (I'm a designer, not a coder ;). Of course, it should be easy enough to do with javascript but a built-in way would be more intuitive.