I happened to notice these entries in the logs while looking for something else:
208.85.175.27 - - [16/Dec/2011:07:04:13 +0000] "POST /wp-comments-post.php HTTP/1.0" 500 2367
81.17.23.117 - - [16/Dec/2011:07:04:14 +0000] "POST /wp-comments-post.php HTTP/1.1" 500 2367
194.8.26.147 - - [16/Dec/2011:07:04:15 +0000] "POST /wp-comments-post.php HTTP/1.1" 500 2367
93.187.17.200 - - [16/Dec/2011:07:04:16 +0000] "POST /wp-comments-post.php HTTP/1.1" 500 2367
213.186.122.27 - - [16/Dec/2011:07:04:17 +0000] "POST /wp-comments-post.php HTTP/1.1" 500 2367
It’s highly unlikely that five people would all be making a comment at one second intervals, so I can only assume that it’s some sort of spam network.
And sure enough, these IP addresses appear at regular intervals making similar attempts, all of which fail.
Remember: All spammers must die.