It seems like you have to interact with github.com to initiate a pull request. Is this so?

UPDATE: The hub command is now an official github project and also supports creating pull requests

ORIGINAL:

Seems like a particularly useful thing to add to the hub command: http://github.com/defunkt/hub or the github gem: http://github.com/defunkt/github-gem

I suggest filing an issue with those projects asking for it. The github guys are pretty responsive.