based on http://highscalability.com/blog/2013/1/28/duckduckgo-archite...:
The fat tail queries go against PostgreSQL and the long tail queries go against Solr. For shorter queries PostgreSQL takes precedence. Long tail fills in Instance Answers where nothing else catches.
It seems that Bing is now a part of their sources indeed: https://help.duckduckgo.com/results/sources