Uber IBM Blogger, Ed Brill, has been impersonated (imwebonated?) on his widely read, highly active blog. Many blogs go through strict editorial controls before comments are allowed through, most however inform the blog author of the post and then let he/she check them out. It does not take much thought to see the potential consequences on security / stock market / personal standing that such a fake blog post could have.
The web is like every other part of society, if you feel compelled to leave your front door open then do not be surprised if eventually somebody comes into your web house and steals something valuable.