Last month, I attended the first .NET language symposium held by Microsoft - Lang.NET 2006. You can read about day one, day two and day three. I'm happy to report that they now have the videos of the presentations freely available to download. Reading my daily reports might give you an idea of what you want to watch, or reading the speakers page itself.
However, I strongly recommend the one on Second Life, simply because it's both entertaining and mind-blowing. The first half is a higher level overview of Second Life and what it all means (very mind-blowing) and the second half is one of the most impressive uses of the .NET system I've seen in a long time. In a previous life I wrote a product that dynamically modified .NET code as it was loaded in, so I can say that what they've done is very non-trivial and very impressive. Wish I had the chance to have done that!