A majority of the search results are from Bing, blended with some other sources (Wolfram, Wikipedia, others).

From the FAQ:

http://duckduckgo.com/faq.html

Where do you get your results?

From over 30 sources, including DuckDuckBot (our own crawler), crowd-sourced sites (in our own index), Yahoo! "BOSS", "embed.ly", "WolframAlpha", "EntireWeb", "Bing" & "Blekko". For any given search, there is usually a vertical search engine out there that does a better job at answering it than a general search engine. Our long-term goal is to get you information from that best source, ideally in instant answer form.