Conditional choice list fields

We need the ability to have one choice list field present a list of values that depend on a selection that was made in another choice list field in the same record. 

For example, I might have a choice list field in an idea that asks the idea submitter to select the area within the product the idea relates to. Another choice list field then requests the user choose a more specific area (from a list that is based on the first choice selection the user made).

Now I can produce a report that shows, at a high level, the percentage of ideas submitted over the general area of the product. And I can also have a report that 'zooms in' to an area and shows the general distribution of the ideas within that one area of our product.

Right now, the second choice list is the entire list of possible sub-product categories, which means we can get invalid data which reduces the value of the reports and the data.