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I recently switched to Ubuntu 11.04 from OpenSUSE 11.4, and when I go to run a project I coded a couple days ago under OpenSUSE using the multiprocessing library, it hangs when it did not under OpenSUSE.
Specifically, I am using two queues, work_queue from which the children get jobs, and results_queue where they place their results before calling JoinableQueue.task_done() and grabbing the next result. I use the "poison pill" technique to terminate the children, where a None object is placed at the end of the queue for each child, and when they get one of the terminating objects they call task_done() again (to account for the None object) and exit.
In the main process, after spawning all of the children (one per physical CPU), it joins with the work_queue in order to wait for all of its children to finish.
This is pretty much a cookie-cutter multiprocessing implementation that I've used successfully for years under OpenSUSE, but for some odd reason the exact same code does not work under Ubuntu.
I would try porting to python 3.x, but the rest of my research team is still using 2.7, so that's not really an option right now.