John Mackey CEO of Whole Foods Market did it for eight years successfully and got caught. Cases reported often hide the fact and underreport the total number of occurances which may be found in the real world. Blogging annonymously in favour of one's own company and attcking the competitors strategies and business models. In other words you can buy media through PR firms but not able to buy the Blog Media. Or can you do it by assuming fake identities and attacking your competitors while creating a viral good word of mouth about your own company.
Although unethical but who cares about ethics now-a -days. So an important lesson in corporate marketing through blog is assume annonymous identity and become your own companies staunch supporters.