I wouldn't have noticed that my site was hacked if Google didn't sent me an email letting me know that he found some hidden keywords in my pages and it breaks their rules and some pages were taken of the index.
I took a look at my FTP and in that particular domain indeed there were some extra files like this:
1. a folder named .xdata which contained at least 300 html files with weird names like: 790-sports-animal.com.html; 2010-heisman-odds.html; agua-bella.html and so on. These html files contained urls and keywords
2. a file named Iog.php which contained the following code:
There is a free program called Agent Ransack that can search though files in a given folder very quickly. I use it at work whenever I need to do a mass search. Download all the files from your site and do a search for the keywords and clean or delete the files.
Wipe the site (this will require some downtime) and upload the cleaned files. If this still persists, check your database and then the server itself.