Strategy + Business magazine has a nice article on co-creation featuring Cisco and P&G among others. It's a good advocacy piece on getting the customer involved.
I've seen the benefits of this myself. Where as before I might have done the usual focus group thing (get 8 people in a room, show them some ideas and then cry when they don't like them) more recently I've taken a different approach.
Instead of the all-or-nothing, customer says yes/no group, I've used a series of hot housing sessions. In these I present some rough thinking to a group (no ads or cardboard) and then get them to work with us on refining and adding to that thinking. I then develop these ideas in the hour after the group and take this evolved thinking into another group. I repeat this exercise over a day or two.
The result is battle-hardened ideas that have benefited from customer creativity and insights. These ideas then form the basis for the creative ideas that follow.