At the Hadoop User Group meeting last evening it was quite the Mahout love fest. First the Yahoo team described their spam fighting efforts which apparently are partially anchored by frequent item set mining using Mahout (thanks to Robin Anil's hard work as a GSoC student). Then later Ken Krugler demonstrated some of what you can do with web-mining and Mahout.
These are just two of a growing number of real-world uses of Mahout which is really beginning to grow up. Related to that growing up, the Apache board just decided to make Mahout a top level project. That will take a little while to make real, but the first steps are already happening.
You can find out more about how Mahout is being used by perusing the Mahout wiki.
Slide decks available here for the HUG presentations.