- If you want me to answer your question and you're fairly sure it hasn't been answered in archives, email it instead of putting it in the comments, as I'm less likely to miss it.
- This is off-topic, but if you live in New York City and you get in a cab with a TV screen, shut off the volume. It's an inaccurate weather forecast and local fluff news anyway, and they only do three or four editions of it in a 24-hour period, so the cabby has probably heard it 10 times before you got in. I asked a cabbie today and yes, it's driving him mad. Do cabby a favor and talk on your cell phone instead. At least that's new material.
So, I have a book finished, and the first chapter would make a great short story. If I submit it to you, should I tell you that I'm also shopping around the first chapter, or should I hold off until you've had a chance to look at it?
So while it is kind of neat to have had the first chapter published as a short story and it builds your writing resume, there are legal issues here. When the magazine purchases your story for print, they buy the copyright for a stipulated period, usually a year. That means that we can't sell your novel if it contains the short story to a publishing house without some difficulty because there's a copyright on part of the novel and you'll have to wait for the contract to expire and the rights to publish the short story/chapter to revert back to you.
It is possible to get the first chapter published as a short story when the novel is sold and coming out; some people do this for publicity reasons, but the publishing company is involved or at least aware of it and not bothered by it. So I would say: Don't shop the short story and the novel at the same time at all, much less mention it to an agent.