Recently we've seen a rash of poisoned search results in google's cached version of the page.
Basically, there's an exploit that allows an attacker to inject code into your website that responds to the "google-bot" and only the google-bot. You'll see your website looks normal when you type in your normal url (http://www.mysite.com for example) but if you do a search in google (and apparently only google), your cached version of your site is a great big ad for Viagra or something equally off topic. This will not be good for your relationship with google, you're helping spammers increase their page ranks (at your site expense) and it looks bad.
If you are feeling industrious, you can fix this problem yourself. First, figure out how the attacker got in. Usually we are finding that there is a FTP site or some other way to access files on your site. Once you've determined that (and changed passwords, updated to latest patch revisions, etc to keep the attackers out), you can do a search on the compromised text and determine which file is affected. You may need to use something that can search inside each file (we use grep with other search tools quite frequently). Find the file, remove the injection and then you're ready to contact google and ask for a resubmission. You do that by using webmaster tools. Here's a link.
If you need assistance with this problem and don't want to do it yourself, feel free to contact us at the number below. We can probably help clean this pretty fast and will get you back up with a minimum of expense. Contact info follows:
