Since splogs get their content from other blogs and simply repost it, I have developed (and just released) Digital Fingerprint, a plugin for WordPress that aims to help blog owners locate instances of unauthorized content use.
Basically, you create your own personal search phrase that is unique to your site alone (think googlewhack) and then insert this phrase into your RSS feed on a post by post basis. If you ever find this phrase outside of your own site, you know you have been ripped off by a content scraper. The plugin utilizes several methods to help you find your digital fingerprint.
Suggestions and resources on what to do after finding out your content has been misappropriated are also included in the management portion of the plugin.
You can download Digital Fingerprint (beta) at MaxPower. I hope it will really be useful for blog owners.