Strange that no one's done it up to now, but former Linden Lab staffer and pal Reuben Tapioca just crunched the simple numbers behind Second Life's current growth rate, and came out with some startling results:
By this time next year, if current conditions hold, Second Life will easily have more than 50% the subscription base of World of Warcraft. That is, nearly 3.5 million to WoW's 6 million. And sometime in 2008, therefore, Second Life will easily eclipse it.
Mr. Tapicoa does the math here. (By way of full disclosure of interest and coolness, Reuben's metaverse development company Millions of Us is sponsoring the Julian Dibbell appearance I'll be hosting.)