Blogs written pretenting to be someone working for a company in order to satirise them are both funny to the reader and potentially damaging to the brand.
The secret diary of Steve Jobs
http://fakesteve.blogspot.com/
This quite clearly is not the blog of the Apple CEO and is very tongue-in-cheek
However, some blogs tread that fine line between being either an absolutely awful company blog or a great parody.
For example, I'll leave you to decide if this really is the official American Airlines blog or not:
http://aaconversation.blogspot.com/
Notes:
For more funny fake blogs, visit Newsgroper:
http://www.newsgroper.com/
This site has lots of postings from 'supposedly' real bloggers (including Al Gore & Briney Spears)
Also take a look at my previous posting about other fake UGC content.