The item similarity code example that uses the api has 4 python streaming mapreduce steps with custom reducers:

http://elasticmapreduce.s3.amazonaws.com/samples/similarity/...

AWS just put a pointer to the code bucket on s3, I'll add a link to a zip file with the full source including a script for converting the raw netflix data to something more friendly to Hadoop streaming.

The article is here:

http://developer.amazonwebservices.com/connect/entry.jspa?ex...

Check out the resources link at the end, the Cloudera Hadoop Hacks site has a few good intros to custom reduers in java, ruby, and python.