I have been in SL for about a week, and it kind of reminds me of the old Palace site. Better graphics and crafting tools, an economy and ownable land, but essentially the same kind of chatting activities. So in a way it is a more tactile version of the web, sitting somewhere between the pure documents format of the web and the real world. There is something kind of interesting of how you can do a risk free 'test' of something in SL while still deciding whether or not to go with it in real life. (whether it is trying out that T-Shirt or haircut to see if you like it, all the way over to playing around with cross dressing.)
So from a retail marketing angle that is interesting, in that on the one hand it lets people try before they buy and see what they think of goods that impact their projected persona... "Those boots look cool, but when I wear them around, I realize they make me look like a tramp. I just saved myself 300$" But for retailers the hope is that they try something risky in SL, get comfortable with it, and then go on and spend the money in the First Life.
The Risk for retailers is the idea that SL satisfies a low level desire on the cheap. Say I lust for the new iPhone. Normally I would wait until it comes out, and then run out there and drop $700 for it, and carry it around until the next cool gadget comes along, and I drop it in a drawer. This is a principle of idle lust that fuels a lot of the impulse purchase type products. But if you get one for your SL character, and get to carry it around in game, this might actually sufficiently scratch the tech lust itch enough that you don't see any purpose in spending the cash in real life. If you want something just 'for show' then to you it may be almost interchangeable to impress people in SL as it is to impress people in real life, and the former is much cheaper.
So for retailers who make function oriented goods, (like say Dell computer) they could leverage SL with confidence. But for image oriented goods, (Like Nike) they would have to worry about demand being sapped through this virtual desire satisfaction.
But the ones who should embrace SL and have the most to gain would be the retailers of goods that have an image that is considered risky or daunting, or requires a big commitment. (a prefect example is getting a tattoo) They are the ones have have a hard time getting customers to try it out in real life, but in SL where the risk threshold is so much lower they have a middle step that lets people get comfortable with it before having to commit. "How do I feel about having that skull tattoo on my arm? let me put it on my SL character for a few weeks, and see how I feel about it then...."