Anyone that has been involved in SEO has heard Google can penalize you for having hidden text on your page, but when it comes down to it, it’s really not anything to be overly concerned with. Unless you are hiding in your alt tag images the words Viagra on the cheap or Texas hold em, you’re usually not going to have an issue with Google. An easy way to see if you have harmless CSS or any other disguised advertising is to use the Web Developer Toolbar for Firefox. Click CSS then Disable Styles then All Styles and you’ll see if you have hidden things the search engines (especially Google) doesn’t like you to have on your page. These are actually things Google could potentially ban your site for or even put your site in its sandbox.