Interfaces to abstract classes

I cracked open the First Look book for the first time since I actually had the bits (three builds later, actually). I was surprised to see that the MembershipProvider was an interface at press time, and not an abstract class as it is now. I wonder what the reason for this is?

I haven't exhaustively looked at the members, but I don't see anything implemented in the base class.

Anyone have an academic point of view on why you'd use an abstract class instead of an interface?

It's kind of frustrating to this point that MS hasn't posted any useful documentation on the PersonalizationProvider class.