DuckDuckGo has been using Wolfram|Alpha (via their API) for about two years now. I actually was an alpha tester and authored the Perl API binding. I've also open sourced the regular expressions I use to determine if a query is relevant to send Wolfram|Alpha. Needless to say, I'm a big fan.
So I am very pleased to jointly announce that we are now official partners. As a result, you should see an even tighter integration over time. And I will have a more official responsibility of maintaining the Perl API, which is in need of a V2 update (feel free to submit bug reports here via CPAN).
Most of the recent integration improvements have come via user suggestion, e.g. I run this or that query regularly on Wolfram|Alpha so can you make that work on DuckDuckGo? I'm in the process of going back through the Wolfram|Alpha examples as well as asking them directly for ideas, but please keep them coming as well!