Adam wrote a small module, inspired by cloudera's MRUnit, that lets one easily create unit-tests for dumbo mapreduce tasks.
The code is currently at:
He also added a unittest for it that serves the double purpose of unit-testing the unit-testing itself
as well as serving as example of how to work with it:
The nice thing about it is that it takes care of some things behind the scenes, e.g deriving the mapper/reducer classes from mapredbase when needed, making sure input/output is iterable (allowing for arbitrarily large input/output test cases - need not fit in memory as seems to be the case with MRUnit), and so on.
Just merged this into my master branch.