Second Life is of interest to me on several levels. It has some game-like aspects, of course, and one can't neglect that it's fun (without having any particular goal) to hang out with online friends, just as IRC can be fun. And I do agree with those who think that some form of the 3D internet will take off over the next decade. While Second Life may not end up being the winning horse in that race (Linden Labs has plenty of opportunities to fail), it's one of the contenders and a good place to get some idea of what might work.
But from a business point of view, right now, it's also a place where real people make real incomes in real money. There are two distinct ways that this happens. First, there are around 100,000 paying customers (never mind the nearly 7 million signups; many of those don't stick around) putting hard currency into the Second Life economy. Linden Labs does manage to take a house rake on every transaction that involves converting real currency into Linden dollars or back, but much of that money comes back out again. The paying population is the size of a medium-sized real life city, and that's plenty big enough for some people to be making a living selling virtual stuff. Thanks to my wife's jewelry business, our membership costs are already covered, and we're on our way to being cashflow positive (not quite there yet, because we've invested in virtual land, but we'll be there well before year end and making extra income as well).
Second, there are consulting companies - "sherpa firms" - who make money by billing real money directly to first life companies who have come to the conclusion "Hey, we need some of that Second Life stuff" and who have no idea how to go about it. Creating a place in the world for the company down the street is the equivalent of building web sites a decade ago, and we're seeing a similar land rush just starting up. Right now most of this activity is big firms paying big dollars, but I expect we're going to see some commoditization over the next year or two. It's possible that one could make a decent living doing four-figure (US dollar) Second Life setups for small and medium businesses.
Anyhow, that's the basics. If you want to poke around, you can try out Second Life for free. If you do get in-world, you can find me by searching for MikeG1 Schumann and I'll be happy to chat with you there.