I just blogged about the How To Integrate Design in an Agile Backlog over at the Pivotal Labs blog.
WHEN SHOULD I USE AN INTEGRATED BACKLOG?
An integrated design backlog is appropriate for small- to medium-sized projects (i.e. teams which can be fed with two or fewer pizzas), focused on a single platform. It can accommodate teams which have designers that code, as well as designers who don’t.
Check it out, and let me know what you think in the comments!
UPDATE: Go read Part 1, How to Manage a Design Backlog and Part 3, How to Maintain Separate Design Design and Development Backlogs.