I don't believe that the high CPU issue is a Channel9-specific issue.
I've had this problem with the Silverlight plugin from the get-go.
A middling-size video in a page will jump CPU usage on my laptop (2 GHz Pentium M, 2 GB RAM)around 10%; full-screening it causes the fan to flare up rather loud and CPU usage to max out. Anything dynamic - a game, etc - becomes staggeringly slow, and misses
terrifying amounts of frames in full-screen. I've had it de-sync the audio from the video.
This could be due to installing both the 1.0 and the 1.1 Beta at the same time, I suppose. I haven't done any testing.
I do hope it's not a Silverlight issue, though.....