Mix08 is sold out! Official announcement here. I am lucky enough this year to be attending in person (and it is also my first time to Vegas). Shoot me an e-mail if you will be out there as well and would like to meet up.
For those of you who cannot attend in person most of the sessions will be posted online like at previous Mix events. This is one of the best resources on the web and I highly encourage you to check it out. There are some really great ones on Silverlight, WPF and Expression.
The search is pretty good too, typing in "Silverlight" will give you a bunch of good sessions.
Check it out here.
See you at MIX!