From Google webmaster tools: "...Dear site owner or webmaster of ...
As of the last crawl of your website, you appear to be running WordPress 3.2. 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 already updated to the latest version of WordPress, please disregard this message..."
They also made a blog post:
Official Google Webmaster Central Blog: 'New software version' notifications for your site
So, if I prefer to stay with older WP version - will it affect my google ranking?