Domain Driven Design
I've been talking to a few friends for a while about adding object modelling into XP stories and having an overall view of the project held as an interaction diagram (often called a storyboard) and backed up by an object model that describes the problem domain.
Anyway, when I mentioned this to Richard Watt and Martin Fowler of Thoughtworks they suggested that I take a look at a book due out soon called Domain Driven Design, Tackling Complexity in The Heart of Software by Eric Evans which talks about a very similar concept.
Evans talks about the use of a single Ubiquitous Language that is used by the whole team - customer and all. That is, you educate your customer in Object Modelling, to some extent.
Well worth a read.