In my fourth blog post of the day, Linden Labs has Open Sourced the client for Second Life in a blog post entitled Embracing the Inevitable.
Linden Labs always said that Open Sourcing the code was part of the long term plan, I remember an interview on LUGRadio a while back. It's a shame it's only the client and not the server-side code, but they say they are staying open minded about that. One step at a time.
My dream (as I described in March) would be a distributed system where anyone could set up their own server. It would use web standards and would just be like a collection of 3D web pages in X3D. It might be difficult to attain the same kind of user experience you get with Second Life, but it would be a great extension of the web.
Update: I've started a wiki page posing the question “What would be required to create a 3D web with a similar user experience to that of online virtual worlds like Second Life?”. You can log in with username:iwontspam password:ipromise or start a new account. I'd value input.