Simon is correct. So far, we've had the following:
Preview 3 is coming relatively soon.
The difference is that the Preview releases are released on ASP.NET, are "official" preview releases. They come with QuickStarts, videos, etc...
The CodePlex source releases are only released on CodePlex and are just meant to show the progress of the project. Think of them like a daily build, only they come out every few weeks. ;) They are much lighter weight, less supported, and don't come with documentation and videos, etc...
Phil