This might not be open source but it is FREE. If you are tired of being charged $1.5 by your cellular carrier for every 411 call, try Google's Voice Local Search. It is still experimental but we know what Google means by experimental or beta. It's usually pretty darn good. Although it was released earlier this month I just had a chance to try it today and I it worked just fine. About 75% of my attempts worked, when I'm not lucky I just say "back" and I get another chance. This service doesn't include residential requests but you can use it to find businesses. You can try "San Francisco California" and "Mas Sake Sushi" and it'll connect you or just say "text message" and it'll send you an sms (both free of charge).
This is a $7B/year market, more than 2.5 billion 411 calls are made every year in America. AT&T is toying with the free 411 calls but competition for this service comes from Jingle Networks which seems to have captured more than 5% of the 411 U.S. market in 2006. Another serious player is TellMe which was recently acquired by guess who... Microsoft of course.