I stumbled across a recent Slashdot posting about Second Life. There are some interesting comments amidst the dross, worth a read if you're interested in virtual worlds or Second Life in particular.
The gist is, Second Life has made its client side open-source, and the article claims it'll be doing the same for the server side. Very interesting for someone who wants to research distributed systems and distributed system security. Getting the servers correctly federated and working well together is an interesting enough problem.
If the client code and the server code are both really open sourced, it may be worth a peek to see what would be required to make a P2P implementation. No doubt there are a plethora of legal and logistical requirements for servers to support monetization and integrity of the world, but heck, that just makes the problem that much more interesting!
While looking up pointers, I stumbled across a tech special report from business week online which is centered around 'Virtual Life,' with lots of Second Life articles. Looks like time for more reading...