If you have used the Silverlight 3 beta documentation on MSDN, you might have noticed that none of the new topics or APIs appear in search results, regardless of whether you are searching with MSDN or another search engine. This is intentional - none of the Silverlight 3 beta documentation has been indexed for search, and it will not be indexed until the official Silverlight 3 RTW release. Therefore, the only way to get to the beta documentation on MSDN is to navigate to it in the table of contents.
The reason we are doing this is because we want to avoid confusing users who are developing for Silverlight 2 only. We do not currently indicate which Silverlight version is supported by each API for the simple reason that each new version effectively replaces the previous version. If you are in a Silverlight 3 beta topic, we assume that you got there intentionally and that you won't mistake it for a Silverlight 2 topic (although there is also some boilerplate at the top of each beta topic indicating that it is pre-release).
Unfortunately, the lack of Silverlight 3 documentation in search results can create the impression that the documentation does not exist, or that there is a bug preventing the expected behavior. Of course, much of the Silverlight 3 documentation is still under development, but you should be able to find many conceptual topics, and at least brief summaries for almost all of the reference topics.
If you are a heavy search user, I recommend that you download the CHM of the beta documentation instead of using MSDN. The CHM search works as expected. For more information on the CHM, see the following blog post: http://blogs.msdn.com/silverlight_sdk/archive/2009/03/18/silverlight-3-beta-offline-msdn-docs-chm-is-available.aspx
For information on the topics we have released with the Silverlight 3 beta, see the following blog post: http://blogs.msdn.com/silverlight_sdk/archive/2009/03/18/silverlight-3-beta-released.aspx. We are also planning to release preview versions of additional topics on this blog, so you won't have to wait for the RTW release for some of the remaining documentation. Of course, there are many other blogs that cover new SL3 features, so a general Web search should also help you if you want to explore some of the features that we haven't covered yet.