There are several infections and compromises that have been affecting WordPress sites. The following links are to tools and information that I used to clean many sites.
http://redleg-redleg.com/file-viewer/
This tool lets you see what your site looks like to the search engines. This is important if you get the infection that uses your site to boost rankings of other sites. The average user will not see this when browsing your site, but they WILL see it if they use a search engine to find you.
http://25yearsofprogramming.com/blog/2010/20100315.htm
http://redleg-redleg.blogspot.com/2011/02/pharmacy-hack.html
Great information on how to identify and remove infected PHP pages on your WordPress site.