What's the use case for not wanting to use an extra FD?

Nevertheless I'm fine with using a function to pick the default selector (but it requires some changes to asyncio too, which currently uses DefaultSelector).

Something I would find useful would be a way to override the selector choice on the command line.  I currently have to build this into the app's arg parser and main(), e.g. http://code.google.com/p/tulip/source/browse/examples/sink.py#64