My issue is similar. I am seeing a user WordPress in my wp_users tables. I am also noticing files like akismet.cache, readme.old, and some .bak files in my wp-content/plugins folders.
I also noticed that the password hash for real users in user_pass contains something like this $P$BQdFLh9rDEjxAY4.JJag7IcIhI2vbQ1 the initial $P$B is repeated in all wp_user tables for all my wordpress installations.
My folder permissions were also changed from 755 to 777.
Before cleaning up my blog which would be a very lenghty process. I would like to know if any one knows how it happened and how I can stop it from happening again. I dont want to go through the cleaning process and find out that the problem is still there. Please help me solve this issue.