That's your problem right there. Most of these attacks are carried out by automated scripts that look for known vulnerabilities in older wordpress systems. Since anyone can look at bug reports and changelogs, it's not too difficult to engineer a script to exploit a weakness.
Your best defense is to always have your wordpress version AND your themes/plugins up to date.
I used to have this problem with a few of my defunct blogs, but keeping them constantly updated fixed it.
Do a grep on your existing blogs and look for any iframes or eval method calls in your WP directory. Also check the DB. Once it's all clean, update your WP version and themes/plugins and keep it updated.
Next login to Google webmaster and, if you haven't already, prove ownership and ask for a review of your site. The warning should go away after awhile.