I have spent the last day fighting with a blog having precisely this problem. It was writing multiple files to the tmp directory and ended up taking down other wordpress blogs on the server as it cut down on the available temp space. We ended up backing up the database and then installing a clean copy of wordpress and re-integrating the database.
The user was running multiple plugins, although I’m not sure that akismet was one of them. The tmp files contained different information, some looked like those described above, and others had entire page (word chosen carefully) content.
Even passwording the blog directory didn’t stop the process once it had begun, which gave rise to serious concern.
As I say, we have headed off the issue by installing a clean copy of wordpress, but this was a very serious problem, which is why I’ve added information to this old thread.