How do designers fit into Scrum and the Agile development process?

Last month we hired a new user experience designer .  We're pyshced to have him on board, and he's already contributing tons of useful stuff in various areas: usability testing, visual design, user interviews, mockups, wireframes, and more.

We use the Scrum framework for agile product development, as I've mentioned in the past.  It's a great framework that I love and that has yielded tremendous results for us.

I'm curious how people combine designers, with their typical wish to have specs and user requirements known up front, so they can do mockups and such, with the small user stories that evolve throughout the sprint.  Often there is not as much time for user experience design or mockups.

How do you handle this?

I've been doing some reading on the topic.  So far I've found a couple of good resources on the topic.  Twelve emerging best practices for adding UX work to Agile development is a fantastic article, over on AgileProductDesign.com.  Another good one isAgile Designer / Developer collaboration with Scrum, by Allen Manning.

Do you knowany other good resources?  Have you run into trouble incorporating more design and user experience work into your agile process?