Blog Hacking?

I just had this problem tonight. I noticed that there was spam on my blog that should have been caught in the moderation queue, and when I checked, all of the common spam words had been removed from the moderation box. Further checking revealed there were two new users on my blog, that e-mail notification to me had been disabled, and that the box to allow anyone to register as a user was checked. I’m almost certain that I had unchecked that last option when the blog was set up. Has anyone else had this problem? I’ve already changed my password as a precaution, but I wanted to get some opinions about this too.