Your're right that you can set it up to select only certain attributes for certain products, however for me, the objective is to select different "sub-attributes" after a first attribute is selected.
For example, for a certain style, if the buyer selects color white, the dropdown list for size should contain S, M, L, XL. If user selects color brown (a less popular color), then the size dropdown list should contain only Medium.
Note that it's not feasible to join/concatenate color and size for an exhastive list of color-size combinations, since this will make the list too long.
I know that Deborah is away. Can anyone else answer authoritatively if this dependent/related attributes functionality either exists or is being worked on? If not, then I'm prepared to code it using DHTML/javascript within the PHP.