> Ah. Then, a documentation error. The error message ("queue.Full"?) and 
> the documentation are totally not clear about that.

Once something goes wrong you are likely to get a cascade of errors, and the first one reported is not necessarily the original cause.

> Does ProcessPoolExecutor just not require tasks to be picklable in Unix?

On Unix the main process forks using os.fork() when the executor is created.  The forked processes inherit all the definitions previously created in the main process.

> Also, this raises questions about what exactly "picklable" means, and why 
> it's not defined in the documentation.

Since concurrent.futures uses multiprocessing the following section applies

    http://docs.python.org/dev/library/multiprocessing.html#windows