Hi,
Web Playlists SDK documenation is now live, so all you developers get cracking. Do let me know what providers you write, there are a lot of interesting things that can be done with it :).
See http://www.iismediapack.com/rolebasedprovider for a sample test app written by our team. This is an integration of ASP.net role based provider with Web Playlists to give different experience to differeng category of users.
SDK Documentation - http://msdn.microsoft.com/en-us/library/aa737442.aspx
Cheers,
Vishal