Yes and sometimes you don't even need to break into the sites. When I was doing some "blackhat seo" about ten years ago I discovered a high PR forum in the same niche that allowed anchor links in the username field. This meant that I could I put something like <a href="mysite.com">keyword</a> as the username and it would just appears as "mykeyword" on the forum index page.
Just one of the many ways I've learned to game Google. Social engineering also works well for link building and plenty of other grayhat methods. While I had considered the possibility of outright breaking into systems to place links, I never believed it worth the risk. Not surprised others have started doing it (or started getting caught doing it).