When given the opportunity to upgrade from Silverlight 2 Beta 1 to Silverlight Beta 2, make sure to verify that the controls you are using will be compatible with the new version. Also make sure not to do this the day before a demo.
Take note of the sites that mention Breaking Changes with the new version.
Also note, that once you figure out how to roll back, the site that you need to download the OLD files from might be overwhelmed because today is the release day of the new version.
You will likley have to uninstall EVERYTHING on you machine that references Silverlight because it is tied to the new version.
Finally, figure out how manually extract an MSI chainer file because the validator in it is picking up things that are not really there and will prevent you from reinstalling the old version. See here.