Silverlight also offers IFS. It is based on the .NET Framework IsolatedStorage classes, so the APIs are similar:
IsolatedStorageFile.GetUserStoreForApplication();
returns a FileStream.
In essence, IFS will let you read and write string or binary data to and from the IFS. Think of it as super cookies.
In the Alpha you get 1 MB per ISO store, which is isomorphic to each XAML page/class. This will likely change as will the ability to adjust storage size on a per user basis.
I asked how CAS is affected by Silverlight and the answer was that a lot of that still needs to be hashed out, but obviously traditional zones may no longer be adequate from a CAS perspective.