bill - 5:37 am on Jan 30, 2013 (gmt 0)
Interesting question. I was under the assumption that they were mainly using Bing in their SERPs, but apparently there's more.
DuckDuckGo's results are a compilation of many sources, including Yahoo! Search BOSS, Wikipedia, Wolfram Alpha, Bing, and its own Web crawler, the DuckDuckBot.