I'm interested in "high-level views"
as well as implementations.
Implementations are generally reasonably deeply involved and you probably won't see much talk of them here. The StackExchange software isn't well-suited to that kind of involved discussion, not to mention it would involve a significant investment of somebody's time.
I would like to find out how hard it
is to write such a system
No more or less hard than any other system: it will depend on your requirements and the features you intend to drive. You can write trivial implementations trivially, but non-trivial stuff will of course be much harder. One of the bigger problems is that you probably won't have enough real clients to tell if your system actually scales to the "massive" level that WoW and other games operate on.
MMOs are not magical. Most of their technology is nothing special when taken in isolation, it's simply that a number of smaller, simpler bits of technology are combined very intelligently to allow for larger scale, connected parallel simulations of some shared world state.
I want so set up a server cluster
where every node controls some part of
the game world, with static
boundaries. Players can now move from
one end of the world to the other
without switching an instance or
hitting some "teleporter".
Essentially what you're talking about is simulation hand-off. This can be done quite simply and isn't necessarily an MMO-specific technology (peer-to-peer games tend to support all the same underlying mechanisms to implement host migration). The basic premise is to have each server understand the topology of the servers "around" it (specifically, a server for world node A must know about the servers for simulation-adjacent world nodes) as well as to define a buffer around which you consider a particular simulated entity "close" to an adjacent server.
When an entity enters the "close" buffer, you start reporting it to the adjacent server as well. Once the entity crosses the actual threshold, you send a message to the adjacent server with the full state of the entity and a message indicating that the adjacent server should take over the entity. Obviously you want this to be as reliable as possible.