I agree with Stefano, this can be a huge headache. Since Google implemented the latest iteration of their search algorithms (Panda) there have been serios implications for duplicate content on the web. You now are penalized for the duplicated content and will lose pangerank.
We always recommend all our clients take immediate action against offending sites. The first step is contacting the site. If that bears no results then you should absolutely file DMCA complaints with Google, their hosting provider, and any advertisers on their site.
You should also take steps to prevent the continued scraping of your site. Another good article for reference:
http://www.blueglass.com/blog/content-scraping-prevention-benefits/
HTACESS is a great start but scraping is a business, and the scrapers have gotten very good at masquerading their theft. That is why there are services, such as ours, that can help protect you against site scraping.
Rami Founder, CEO
www.distil.it