"Imagine a Moodle course that, if you wanted, could turn into a proper 3D interactive classroom with all your Moodle resources available to your students in the virtual world." This is very doable, since you can import external objects (such as videos) into Second Life. To me, the greater challenge will be fusing an open source LMS into a commercial 3D environment. But hey, it might work! Then Blackboard will say it invented it.
I would ask WHY? Why do we want a 3D Learner Management System (be it patented commerical or free/open source)? Do you want to create such an environment because you CAN? What are the potential of 3-D in enhancing learners' experience? Please enlighten me.