My most recent article on Mahout is up at IBM developerWorks. It is titled Apache Mahout: Scalable machine learning for everyone and is designed to walk you through using Mahout with a real email data set using Hadoop and EC2. It also gets you up to speed on some of the new things in Mahout since I last wrote on the subject for developerWorks.
Note, I will also be giving a talk on the subject, at the San Francisco Bay Area Mahout User Meeting on Nov. 29th. Hope to see you there!