Recently I had to transfer all the blog posts from an old 2.4 version of WordPress, to a new 3.0 install. After I exported the content (via Tools > Export), and tried to import it into the new blog, the importer froze up and never completed. I was surprised to notice that the XML file was 50MB. Opening up the file in Notepad++ revealed that this was all spam. Apparently the blogs spam comments had never been deleted and there were over 10,000 spam comments!