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.