What Motivates People to Take Surveys?

I came across this post from Business Week this morning which discusses how Sulake used Habbo, a nine-year old virtual world that has over 100 million avatars, to survey 58,000 teenagers on topics varying from cosmetics to mobile phones. We've recently discussed the good and bad side of deploying online surveys, but what Sulake did was extremely clever. He utilized his online virtual world as a platform to question teens. Not only that, but he questioned teens on items that he knew would be of interest to them. The survey took roughly 31 minutes to complete, and the respondents were given an incentive to participate in the survey (complimentary credits to purchase virtual decorations and furniture for their virtual rooms). Sulake knew exactly what it would it take to get the millions of users who log-in everyday to their virtual worlds to take the survey. What incentives/prizes has your company used to entice your customers to give feedback?