On the note of "web-app stuff applied to an MMO server", has anyone checked out NetEase/Pomelo (an MMO engine built on top of Node.js)? For what I've seen it has a pretty straight forward way of encapsulating distributed servers (although it seems to still suffer if too many players congregate in one area). Sadly, I haven't been able to find any real analysis apart from these slides: http://www.slideshare.net/xieccy/pomelo-jsconfasia So I don't really know how it handles server interactio and how would a physics simulation would run in there.
I bet through some clever trickery and using player locality you could engineer a system of virtual boxes or spheres of players that overlapped and allowed you to grow or shrink on demand.
Awesome, I recall thinking about something like that when I "dream of making an MMO some day" in my teen years. I'll have to try it out when the day finally comes.