When you read a blog endorsing a brand or service, how do you know that it's a legitimate blog and not been paid for by the advertiser? It has become a heated discussion especially in the States, where the Federal Trade Commission has recently produced guidelines which say that a blogger 'must disclose any material connections between themselves and a brand that might affect the weight and credibility of an endorsement'. You can read more about it in this Forrester Research article. How someone is meant to disclose this in Twitter's 140 characters it doesn't say but it does give some useful rules when taking part in what they descibe as 'sponsored conversations'.