Would it be at all possible (and I'm sure this is a stretch) to modify the data being sent to a client in realtime with a plugin? You know when blocks are still loading and you can see underground to all the caves and lava which have loaded already, but you can still walk around normally and collision detection still works? Think it would be possible to intentionally alter the feed to a client to make this happen on purpose? For example, a certain distance from the player, or so that only certain block types appear? Would be a great anti-griefing tool, not to mention fun for hunting dungeons and resources (although this would make things way too easy if you gave it to all your users). I get the feeling that we can't really dig that deep into the Minecraft server (yet?) but it would really be something, even for simple things like Realms markers that only the admin can see, and don't interfere with regular players.