Musicians are already playing "live" gigs in Second Life and -- at least one -- is bringing in a nice stream of cash on the side:
CNN: Artists visit virtual Second Life for real-world cash
It seems strange today (one of the ever-popular "can you believe what people do online?!?" stories) -- but this will become increasingly common. Personally, I'm not sure we've seen the tipping point for virtual worlds yet, the numbers of visitors is so small.
The skeptics need to realize that fundamentally the Internet is a medium for connecting to other people -- and that applications like Second Life are just the next generation of online chatting and web broadcasting. With a healthy dose of open-ended play too.
The real question is whether these networked technologies are really anything new or just an extension of the mechanized social interactions of modern life? Guess what I think.