WP.com, to my knowledge, has never had a code or security breach.
There are communities (like Club Penguin) where they are quite promiscuous with their passwords and share login details with each other regularly, and then "hack" each others blogs.
I did the same search. In each of those cases it seems like being "hacked" means adding another admin and having them screw you over or having your password guessed. Though I'm no Wordpress fan, blaming them for things like that is rather absurd.
http://www.google.com/search?q=wordpres.com+hcked&pws=0&...
You'll find threads like these:
http://en.forums.wordpress.com/topic/my-site-got-hacked
http://en.forums.wordpress.com/topic/my-friends-site-was-hac...
http://en.forums.wordpress.com/topic/i-got-hacked-1
where wordpress.com users seek help after they got hacked.