Spam in posts

Every so often I find the post on blog have been spammed with a link inserted every so often throughout all the posts. Good job I have the database automatically backing up every day so I can run it back to the latest good version. Any ideas what this could be?

There doesn’t seem to be any additional users/authors and it’s an up-to-date version of wordpress. The only thing I can think is that a couple of post-to-twitter style plugins I added recently have created a way in. I’ve been turning off the plugins one by one to see what happens.

Anyone else having this problem? All the spam plugins seem to stop comment spam – I haven’t as yet found one for post spam.

M