Special K is the latest brand to be cashing in on social currency with the popup "tweet shop" that launched in London in September to promote a new line of crisps.  People could get a free packet by paying with a tweet. 

I'm not sure how successful the campaign was for Special K - it doesn't appear that the hashtag #tweetshop has any attribution to Kellogs.  What I find interesting is the very open call to action to cash in your social currency.