Colocating the members of the team, Developers and Customers (including Testers), does wonders for increasing the communication on a project. And the trust.

conducted a field study of six such teams, tracking their activity, attitudes, use of technology and productivity. Teams in these warrooms showed a doubling of productivity. Why? Among other things, teams had easy access to each other for both coordination of their work and for learning, and the work artifacts they posted on the walls remained visible to all."