Defining technical requirements for a datawarehouse, an existing one at that

I have no experience doing requirements documentation for a data warehouse, and am hoping an experienced practitioner or two can provide some practical lessons from documenting a data warehouse.

To add an extra twist, the data warehouse is already in place and in use. While this helps to see the functionality in-place, it does present some confusion as to what requirements are real and what are best left to the implementation level of things.

Any tips? References to check out?