Only a couple of slides were used to give a brief overview of the layers that make XNA work, and then David "LetsKillDave" Weller went right to demonstrations. In the course of about an hour, he took the elements provided in one of the tutorials that comes with XNA and developed a fully functional Asteroids clone, complete with collision detection, scoring and separate background and HUD-style overlay elements.
Dave did a good job of keeping the presentation moving, despite a couple of incidents here and there. He explained the parts of the code that he was adding in and when something would get fouled-up (which is always to be expected in a live demo), he would open another version of the project that was "pre-baked" to a different level of completion and, fairly smoothly, continue his demonstration.
The presentation was recorded on video for inclusion in the DVD, and is currently available for viewing online for TechEd 2007 attendees via the TechEd 2007 website.
For more information about XNA development and for related downloads, check out Dave Weller's Blog (LetsKillDave.com) with the link below. Also, I've added a link to video of a demo given by Dave Weller, "Make a Game in 60 Minutes", at the XNA Creators' Club website; this is the same demo as was presented at TechEd 2007, but was performed at a different time.