Dynamic custom fields

Hi All!

My company is making a huge move from a large “proprietary” ticketing system
to RT3.0. One of the things that is require before the move is the ability
to do the following. We need to make one custom field depend upon another.
For instance if you where to select a certain “Category” only certain
"Sub-Categories" would be available based on your choice. Any help would be
greatly appreciated!