[jdswain]

There is a class of tools for this called Requirements Management. They tend to be expensive and clunky tools in my experience, but I haven't used one for quite a few years. Ones I have used are Rational Requisite Pro (IBM) and Doors, which is now apparently an IBM Rational product too. I've worked on a project that used Doors for requirements management but also requirements traceability, a somewhat tedious task that would let us look from a requirement and identify all development documentation that implemented that requirement and eventually to code if required.

From an logical point of view I think it makes a lot of sense to start from clearly documented requirements, then work forward to design, implementation, and testing with links back to each requirement. I don't know how testing (higher level than unit testing) can really be effective unless they have requirements documents to use as their starting point. Use Cases go some of the way to providing this information but tend to be a bit less formal. Agile methodologies tend to be less formal than older methodologies that emphasised this kind of process more. A lot of Agile is good, but it does also tend to forget lessons from the past.