yes you are right many have got penalty as a result of the guest post
o_O Then Article directory sites also looks like guest blogging or posting. :/ why Google didn't penalize all article directory sites? Strange and confusing.:
article directory is never guest posting.
blog supposed to be only unique posts by the owner to his/her readers.
also google has found people now go for higg pr blogs for guest posting, after paiding some good amount of money.