A March 2007 study of 1,085 Second Life avatars by CB News / Reperes found that "brands are still far from having succeeded in exploiting all of their SL potential. For instance no RL brand introduced in SL has succeeded in establishing a strong presence in the minds of residents."
The graph above demonstrates the failure of brands to build strong "spontaneous" (unaided?) awareness. K Zero comments that these low levels, especially for the brands with longer history of SL presence, may be due to the high avatar turnover and the influx of the new residents.
The good news: "66% [of the residents] believe that the presence of RL brands has a positive impact on SL", which contradicts an earlier study by Komjuniti that showed that "72% of respondents [...] as being disappointed with the activities of the companies in Second Life."
A similar report by Market Truths shows that 49% percent of residents think that brand presence in SL is a good thing. Surprisingly, however, "of the 21 brands ultimately named (unaided), four do not actually have an official SL presence. They are, however, still getting positive brand impact from their “unofficial” (or perceived) presence." (quoted from SL Communicators).
But do check out the entire report by Reperes -- it has some interesting resident insights about specific campaigns.
[Update a few hours later] Ha, another study on Second Life: The latest poll by Global Market Insite, "found the virtual world of Second Life is a burgeoning market for real-life brands and product promotion. Fifty-six percent of users believe Second Life is a good promotional vehicle. Only 16 percent say they would not be more likely to buy or use a brand that is represented in the Internet-based virtual world."
Importantly although not surprisingly, " 55 percent watch less television since becoming active in Second Life." (press release)