Make better apps by trying things out first, before you write any code. Get a glimpse of Apple's prototyping process and the range of tools and techniques we use, some of which might surprise you.You can also download it in HD or SD as well as view the slides.
Most of the WWDC videos are developer related, but there are a couple designer ones this year. The one I'm most excited to watch is from the Apple Prototyping team AND THEY USE KEYNOTE TO PROTOTYPE! (I have an overview on how I do this here). It's available to watch online now (you must use safari to view it) and it's called "Prototyping: Fake It Till You Make It" with designers Jeffrey Traer Bernstein, Linda Dong, Julian Missig, and Mark Hauenstein.