On my current project, the business users are specifying their requirements in the form of User Acceptance Test Expected Results. Every Friday afternoon, the head of credit and his three most senior staff spend an hour specifying the expected results for a set of sample inputs.
They are doing this for a number of reasons:-
1. They trust us (I hope) when we tell them that they are important.
2. The can state with certainty that the results are accurate whereas a functional spec is normally too abstract or lengthy to understand fully.
3. Two uber developers Andy Pols ( www.pols.co.uk ) and Joe Walnes ( joe.truemesh.com ) did a two day XP spike where they implemented two of the UAT results using FIT. The head of credit saw this and could see how important the UAT results were to a UAT driven development. It did not need an act of trust because four man days is not a big investment on a multi-million pound development.