This morning I found a very interesting open source project by Sun called Darkstar.
"Project Darkstar is the game industry’s first open source, enterprise grade, highly scalable, online game server, " defined on Darkstar's site http://projectdarkstar.com.
I went ahead a created a user account and then trolled the forums for info nuggets. From the the read of it, DS is in the early stages and is still in single-mode operation. The hop to multi-mode looks like will happen when 1.0 is released sometime soon, hopefully.
In sum, I see great potential for niche virtual world projects besides just gaming platforms. As Sir Gordon Sumner onced yodeled, "I'll be watching you!"