Last January, Cory Linden predicted it would happen by the end of the year. With just two days to spare, it came to pass: peak Second Life concurrency reached 20,000 in 2006.
Other Cory Linden predictions for 2006 that have also verifiably panned out: "Apple's share of the PC market will double to 5%" (Check), "A Second Life resident will begin selling a service for exporting SL items to a Fab Lab... in order to create them in the real world" (Check), and, "A Virtual Research Foundation, based in a virtual world or MMO, will be created to gather games and virtual world research, create research standards, and provide funding to researchers" (Check, kinda sorta), and, "A winning candidate in the 2006 US Congressional elections will have campaigned in an MMO or virtual world" (Close, but no cigar.)
Update, 6:52pm Hawaii time:
In Comments, Cory himself notes another prophesy of his ten which manifested. That's an astounding 50% accuracy so far, and a few others may still be confirmed. Longtime Residents know that Cory Linden often visits the world as the Flying Spaghetti Monster, however, so I suspect some mystic counsel at work (FSM be praised).
Update, 7:21pm Hawaii time: Of course, it's also worth noting that the 20K concurrency came at great cost. And just as surely, Tateru Nino has a post with a title that says it all.