There has been a lot of discussion recently in the press and in the offices of DigiKnow as to the real numbers of SecondLife. CNet has a great rundown which I can summarize and save you the read. While Linden Labs (creators of SL) state a user base of over 2 million users, approximately 220,000 are actually active in the system. 42,400 people pay for the premium accounts in SL. Contrast that with World of Warcraft's 7.5 million paid users.
Despite the controversy, SL has promise. The virtual world allows you to create an avatar (a virtual representation of the offline you) and interact with it. This includes meeting other users, chatting with them, buying goods and services, relaxing, etc. Companies are creating offices, holding press conferences and scheduling meet-ups in SL. These could potentially be very valuable marketing opportunities when talking to an audience who is apt to participate.
These virtual worlds are gaining momentum and marketers are jumping on-board. The real key is to make sure that your audience is using these types of games/experiences. Most likely if you're not targeting men in their mid-30s who still live with their parents, you may have to wait for this to catch on a little more.
Just noticed this article on Reuters with one of Gartner's analysts who thinks the SecondLife hype is near it's peak.