I recently attended a live chat session with the Google webmaster team and here's what they said:
If your blog content is syndicated via RSS feds, Google can distinguish original articles from syndicated content if they have a link back to your site.
You can read the full summary here.
If you can modify your full RSS feeds to include a link to your original blog article, the people who republish your full feeds are actually helping you because Google will have no trouble recognizing the source.
And your regular readers will love full feeds much more than partial feeds.