From Hackathon to production: building a large scale recommendation engine
exchange.causes.com
exchange.causes.com
https://docs.google.com/viewer?url=http%3A%2F%2Finfolab.stan...
http://lucene.472066.n3.nabble.com/Mahout-not-giving-recomme...
But that's also an older article, and things might be better. Mahout might be worth checking out.