Having been a big Secondlife fan for nearly a year know, I have always been convinced that Secondlife can be a place for brands to interact with people in a more engaging way than is possible through a normal browser.
Here are 2 very good lists of Brands in Secondlife. The first which is more visual from Kzero, and the second from Wikipedia.
Perspective: Some of these entries offer an engaging and brand relevant experience, others are a mere presence, but at least they are learning.