There’s a lot of really big, cool things coming out of Mix09:

Silverlight 3.0
Silverlight has been on my list of things to learn for over a year.  Now I’m even more compelled to get up to speed.  The first beta release is on the Silverlight site now

Yeah, it’s out.  Not much has changed since RC2, so if your app runs fine on RC2, the upgrade path should be almost completely pain free.

Microsoft Web Platform Installer and Application Gallery
Hanselman has a nice write-up about packaging your web application up for use with the Web Platform Installer.