While at TechEd we had 65+ people run through my Hands On Labs and we had a great time!
Unfortunately a number of people had problems with rendering the video...the reason was setting the source to the production server was asking the application to make a cross domain call - something it can not do from its security sandbox. The solution in the labs was to add the WMV to the project (Project > Add Existing Item) which in allowed the call to be local -from where ever the code was run from. While this worked for a single WMV that we had control over, this won't be the normal scenario.
The correct way around this would be to set URL in the EnviromentPermission settings in the project in Visual Studio 2008...unfortunately this is broken in beta2
In the meantime you can use the work around Chango Valtchev figured out:
1. Add PresentationHost.exe as a ‘project’ to the solution and make it the startup project.
2. Set command line to be -debug <path to .xbap in build folder> -debugSecurityZoneURL http://whateverhostyou wanttofake.com
3. Changing the debugger type to ‘Managed only’.
4. Run VS non-elevated on Vista. If it’s elevated, it won't work due to mismatching process integrity levels