This is the interesting discussion to have, and the one I am having. And even if the weapon itself is not ready in minutes, it's easy to imagine a world in which printing the open source AK-47 is as easy as: Visit website, search for AK-47, click print on first result, eventually refill relevant cartridges. It may only pop out hours later, but it'll still be a different world than the one we live in.
I completely disagree with anelson's characterization of the difficulty; I could make "printing a web page" look as difficult if I spelled out all the steps ("purchase paper for printer, insert paper into printer..."), but that's not reflective of the real difficulty.
Remember, Thingiverse is not version 7.2.3-final. It's 0.0.1-pre-alpha. We are not at the endgame of this technology, we are at the very, very, very beginning, and even this discussion about guns is merely one high-profile example of the sorts of questions we're going to have to grapple with when the full power of software is unleashed into the physical world.