The only changes are some bug fixes for the .Net version and a couple additions to the SL version.
We brought back PackageCatalog but redesigned it under the name DeploymentCatalog and put it in System.ComponentModel.Composition.Initilization.dll
We renamed PartCreator to ExportFactory and put it in System.ComponentModel.Composition.Initilization.dll
Renamed type PartInitializer to CompositionInitializer
Renamed method CompositionHost.InitializeContainer to CompositionHost.Initialize
This drop contains a MEF.sln which loads in VS2008 and builds MEF against .Net 3.5 and MEFSL.sln which loads in VS2008 and builds MEF againts SL3. The API's are pretty much what we are shipping in .Net 4.0 and SL4 SDK respectively. However if you need a .Net 4 version of MEF we suggest that you install the RC version of .Net 4 and if you need an SL4 version of MEF you install the Silverlight 4 beta SDK.
As aways if you have issues or questions related to MEF feel free to post them in our dicsussion forums at http://mef.codeplex.com/Thread/List.aspx.
For some more detailed explanations of the changes in this preview have a look at Mike Taulty's post http://mtaulty.com/CommunityServer/blogs/mike_taultys_blog/archive/2010/02/17/new-mef-drop-preview-9-on-codeplex.aspx.