All the example use the built in "aggregate," which looks like it refers to the Hadoop aggregate sub-package. The lines output by the map functions start with the name of a Hadoop class within the aggregate package, which must tell it how to aggregate.
http://developer.amazonwebservices.com/connect/entry.jspa?ex...
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.
http://elasticmapreduce.s3.amazonaws.com/samples/similarity/...
You will get much better performance out of Hadoop if you do this in Java, but I thought python would be more readable for a wider audience. Hadoop 18.3 doesn't have numerical sort in streaming (see https://issues.apache.org/jira/browse/HADOOP-2302 ), so the code is slightly more complicated than it needs to be. Dumbo will also probably be supported in the next release, then the code example would be even simpler.
edit: oh, guy above me has great link!