Defining all these methods on the core servers, and also defining "pass-through" methods on the web servers would defeat the point of having lots of functionality, but very little code.

If, on a web server, I say: User myUser = User.retrieve(12); I want it to go get user #12 from the core server. I don't want to have to specify that this method is somehow special because it, like every other method for every single object, is defined within the core.

The other part of the problem is if I want to override "User.retrieve()" locally. That local method should still be able to call it's superclass' method on the core server.

With Perl, this is easy: Just use AUTOLOAD. With C#, it is possible, but not easy. There is currently no C# equivalent. We would have to define the same methods locally (by hand), but have them call out to the core using webservices. If we have 100 servers running as clients to the core, we would have to go update 100 servers with new methods whenever we add new functionality. If our application has 1000 methods running on 100 servers, it can become unmanagable very quickly.

Consider this scenario. You have 30 websites running as clients on 1 server. Website#1 wants their Contacts to have unique email addresses, but Website#2 doesn't. Website#3 wants 1 Contact registration per household, Website#4 wants at least 3.
We can simply have domain-level Class method overrides take care of the Contact.Create() method, leaving the rest of Contact alone.
We could have implemented some front-end logic on their website's lead-capture form, but what about the administrative tool which everyone uses? We couldn't enforce the same validation/requirements on the backend this way.

We can offer sensible default behavior out of the box, but when (not if) our customers come to us with special requirements, we can now accomidate them with a reasonable amount of effort, while leaving our other customers' data, objects and experience out of it.

I would be very interested in finding out techniques for achieving the same kind of runtime flexibility and extensibility using other development platforms out there. In fact it could make writing the plugins for other platforms a bit easier later on.