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This is similar to #10348, but has a different scope; the attached patch disables the ProcessPoolExecutor if the system has too few POSIX semaphores.
To keep support for the ThreadPoolExecutor, I had the test cases stop using multiprocessing.Event in the threaded test cases. Unfortunately, this had two side effect that I think indicate a bug elsewhere: 
1. ThreadPoolWaitTests.test_all_completed_some_already_completed hangs
2. (sometimes) ThreadPoolWaitTests.test_first_exception fails:
 self.assertEqual(set([future1, future2]), finished)
AssertionError: Items in the first set but not the second:
<Future at 0x1851ad0 state=running>
I haven't been able to determine yet why it hangs. If the hanging test is disabled, the tests pass on both Linux and FreeBSD 7.3.

Martin,
Could you commit this patch if you think that it is the right thing? I'm going to be restructuring the tests and don't want you to get caught in merge hell.
Cheers,
Brian

Is this really the time for restructuring tests? I think such activity should wait until after the 3.2 release.

The tests are currently flaky on Windows and I'd like to fix that before the release. But I don't have the bandwidth to debug the Call() abstraction used in the tests before 3.2 RC1.
You can see the test change here:
http://svn.python.org/view/python/branches/py3k-futures-on-windows/Lib/test/test_concurrent_futures.py?r1=87484&r2=87652
So far it is passing on all the platforms that I've tested it on (Mac OS X, Ubuntu and Windows 7).

Thanks for the review, committed as r87665. Fixing flaky tests is fine, of course.

Follow-up: I have disabled test_all_completed_some_already_completed in r87667, as I couldn't figure out why it hangs.

test_multiprocessing is still failing with "OSError: [Errno 23] Too many open files in system" on "x86 FreeBSD 7.2 3.x":
http://www.python.org/dev/buildbot/builders/x86%20FreeBSD%207.2%203.x/builds/1396/steps/test/logs/stdio
I don't know the maximum number of POSIX semaphores on this buildbot.

Interesting. I wonder whether a reboot of the system would help (as it may have old semaphores hanging around).

Is it safe to assume that this test problem was resolved long ago?