OK. Let's say you're working on a textbook scrum project. You've got a scrum master collaborating with a product owner. The next sprint is UI-heavy - by the time your coders start building screens, you really want to have some idea what they're going to look like.
Who does the wireframing, and when? The product owner? Somebody supporting the product owner? The scrum master? If you have a UX expert, do they work alongside the coders after the sprint has started, or do they supply wireframes & mock-ups up-front that sit alongside your story cards and constraints to guide and inform the work the developers are doing?
I'm pretty sure we need some UX help, you see, but I'm really not sure where to apply it...
EDIT: Let me rephrase the question.
How do you deliver consistent, high-quality user experience on an agile project?