I phrased it as a question because I don't know the answer either. I've heard that second life had a thriving economy of user-generated content and that some players "make a living" from the game; I've never seen hard numbers though.

I doubt the returns reach the proportions necessary to support a startup, especially with the low barriers for entry, competition from free alternatives, and piracy. Maybe it could support a single developer, though; more of a "lifestyle business."