I got this email yesterday for a blog that I help my brother-in-law out with. I haven't ever seen a notice like this:
Dear site owner or webmaster of http://domain-name-removed/,
Your site appears to be running an older version of WordPress. Google recommends that you update to the latest release. Older or unpatched software may be vulnerable to hacking or malware that can hurt your users. To download the latest release, visit the WordPress download page.
If you have any additional questions about why you are receiving this message, Google has provided more background information in a blog post about this subject.
Google Search Quality Team
I didn't know that Google would send a notice like this. Seems like a good thing to do to keep the web a better place.
I logged in and upgraded his site for him.
Update July 6th: This seems less cool than I hoped. Today I've gotten a number of emails on blogs that I upgraded on 7/4 to the newest WordPress saying I should upgrade them. Maybe Google is just seeing that the site is running WordPress and sending a note regardless of what version it is currently running?