I have to say that after reading about the successful teleport of a Linden pilot team from Second Life to OpenSim yesterday, and the focus on integration between the worlds, I can't bring myself to believe that any virtual world will actually "take over" another virtual world. At this point I see them all as countries on the globe hammering out the specifics of their border treaties. I also see them as potentially serving different metaverse needs - WoW for playing, Lively for work / business, SL for art and commerce, etc.. and therefore not so much competing with each other. I guess we will see what happens in time.