This could be from any number of sources. The link you provide probably isn't how they got into your website.
However, if you have phpMyAdmin on your site, or osCommerce, Openx, or any number of other software programs, then you should have them updated - immediately. We've been seeing a number of attacks against standard software on websites.
If you have your access logs, look in them for a series of 404 errors where someone is scanning your site and looking for where software is installed. See what file they're looking for then do a search on that log file to see when it returns a 200 - meaning they found it. Usually that datetime stamp will be close to when your site was hacked.
Although, if the hackers uploaded a shell, you can't base your search on datetime stamps. Many of the shells we've been seeing have the ability to change the datetime stamp of files - to further hide their work.