Oren Nachman's session, Optimizing Performance for Silverlight Windows Phone Applications. Oren is a tester in the Windows Phone team who really hammered the platform searching for nasty bugs, and he did find a few.
While Jaime's session gave us a high-level look at the architecture underlying Facebook and Tweeter apps, Oren's session is focused on delivering high performance code and understanding some of the system limitations. If you're not familiar with Silverlight, the session will be somewhat difficult to follow. But if you know Silverlight and you plan on writing a Silverlight WinP app -- this's a must-see session for you.