Some recent announcements show movement on the part of Linden Labs to open up their protocols for Second Life such that others can host servers to participate in a federated, 3D version of the world wide web.
This is not Linden Labs first journey into open source when they released an open source version of their HUD. What's different this time is that they are taking steps toward releasing server code and protocols such that others can host virtual worlds on their networks which you could teleport to using the Second Life HUD. It still has a long way to go but it is a step in that direction.
Linden Labs isn't the only one using open source as a way to leverage themselves into a larger market where they have to share. This larger trend of moving from content provider to content platform provider means that Linden Labs has to give up a share of the revenue pie in order to be a founding participant in the vision of a World Wide Web where web pages are replaced with 3D virtual spaces and surfers who happen to be visiting that place at the same time get to text message each other.
Considering the impact of Web 2.0 and social networking on the web, I believe this to be a very natural and obvious evolution of the web. There are plenty of competitors who would love to beat Linden Labs to this punch so perhaps that is their "bottom line" motivation to move in this direction.