I've been trying to get DDG into the default list of Web browser search engines wherever possible. The big ones (Chromium, Mozilla, IE, Safari) are very difficult to get on a budget, but the smaller ones may be possible.

If you're involved with any of them, doing something like  Mithrandir  did for Trisquel on this thread  may be the way to go. 

That is,  bring it up on the project mailing list or IRC channel.  Clearly the more inside someone is the better.

As far as I know the only ones it is in there for right now are:

• iCab Mobile:  http://www.icab.de/mobile.html . I got this by emailing the (single) founder a few times.
• Tor:  https://trac.torproject.org/projects/tor  thx to __nil 
• Opera:  Right now it is in the US, UK and Int'l English versions. More details at: http://my.opera.com/ruario/blog/2011/10/19/the-hidden-feature-of-11-52-duckduckgo-is-added-to-opera  (from this topic), added on this topic by  James Hunter on 10/19/2011.
• Midori: Default search engine as of version 0.3.0. (added by AGreenEarth, 10/23/11)
  
• Iceweasel: in Debian wheezy (from this topic), added on this topic by James Hunter on 10/19/2011.

Other good targets I know of (I'm sure there are many others, and linux distros count too):

• SeaMonkey
• Avant Browser
• ThingFling 
• Camino
• UZBL
• gnuzilla
• epiphany
• Epic Browser
  

As for the big guys:

• Firefox - talked to the right guy, but isn't interested at our level (probably wants rev share).
• Safari - no contact
• IE - no contact
• Chromium - asked on the mailing list; they said it is the top 3 in each country.