Having developed many websites myself, one of my greatest fears is people stealing my original content. I know you can use services such as CopyScape.com to find out who has stolen your content, however what action can you take against them?
This one is a tricky one! The sad news is that not much can be done with out proper legal representation. This is why many publishers simply deal with it because they do not feel like hiring a lawyer. If you find out who high-jacks your material I would try contacting them to take action.
Good topic! I don't know what I'd do if someone stole content from any of my sites. But thankfully my brother's a lawyer.
If your site is doing really well though, it might not really matter that much. Most successful sites online are unique sites. Sites that copy them don't do half as well really.