The official Google forum/website has announced that if you do not allow the Googlebot to crawl your CSS and JS files, your rankings could be impacted.

This basically means that if you are disallowing the Googlebot in your robots.txt file, you might be impacted.

More info here:

http://www.clickthrough-marketing.com/seo-google-webmaster-guidelines-updated-says-dont-block-javascript-or-css/