USA Today had an article about this recently. They interviewed one guy who makes $1,000 a month selling skins (character textures) for Second Life.

And someone posted here awhile back, asking for help with P6. She eventually admitted that she didn't really want to learn Poser at all. She just saw other Second Life merchants converting Poser clothing and hair to Second Life format, and making big bucks. She wanted to do the same.