Lou Rosenfeld shares the questions one should pose when engaging prospective consulting firms for information architecture and information management project work. This list of questions should be very valuable if you are on the customer end of a client-vendor relationship.
It also seems like it would be an interesting list of questions to think about if you're a consultant and have never been on the customer end. As a consultant, you probably already ask many of these questions of prospective clients as well, as part of your first client interview about a new project. If you're on that end, this gives an idea of the kinds of things to be prepared to answer.