I have been reading up on the new features in Silverlight 3 over the last couple of days, and have often been confused because what I was reading didn’t line up with what I was seeing. It appears many of the articles and videos that are currently available have been created against the Silverlight 3 Beta version, and a number of changes have been made between the Beta and final release. For example, the DataForm control is often mentioned as being in the System.Windows.Controls.Data.DataForm.dll assembly, but has actually been moved from the Silverlight SDK into the Silverlight Toolkit.
I found this page on MSDN that lists all the breaking changes in Silverlight 3 has clarified many of the issues for me. It details the breaking changes from both the Silverlight 2 and Silverlight 3 Beta versions. I’m sure that as time goes by more articles will be referring to the release version and things will become less confusing. In the meantime, if what you are reading doesn’t seem to add up, I would start by checking out the list of breaking changes.