Not so long ago, Ilya featured an awareness study on brands in Second Life. But as the last time I checked Second Life was still supposed to be about conversations and communities, the real proof for brands lies in "how long do people actually hang out in your wonderful virtual building".
The slide I attached to this post gives some insight into this (click to enlarge).
It is based on an excellent analysis by Gary Hayes over at Personalise Media where he compared the traffic or "dwell" at a number of key brand locations in the virtual world. This is a relative measure operated by Linden Labs which creates an index based on the number of visitors and the time they hang out on a piece of land (for a fuller explanation, check out this post by Sue Baskerville.)
Gary also made a nice video compilation of brands in Second Life.