In the past, everyone knew that Google and the other engines, stayed away from your CSS files. However, after on October 19th, Matt Cutts posted a blog entry named SEO Mistakes: Unwise comments which showed an example of someone using CSS. Matt wrote;
I don’t recommend that people use CSS to hide text, and I don’t recommend that they document it, either.
Then Jagger update hit us, and many now believe that Google does peak at your CSS files. A DigitalPoint Forum thread asks Does google read external CSS files yet? Some reply "no" right off the bat, but I wouldn't be so quick to say no. Many respectable SEOs have been chattering about Google reading your CSS just for this. So just be careful, especially now.