I genuinely and honestly cannot log into your Gandi account and fix your nameserver delegation, so that means I'm just here to shit on things? That's a logical leap for you? You are delegating xip.io to a nameserver that is refusing queries for your zone; that's seriously broken and can result in resolution failures, making your clever hack worthless.
I don't know why I bother providing feedback, since people from your school of thought (I'm looking at the 37signals community as a whole, here, which you're being a shining steward of) just get defensive and take your software being broken personally. You wrote a poor DNS server. Read the spec, study BIND's or NSD's source to understand the years of work that went into this before you, and understand the problems I've pointed out. I just get annoyed when people flagrantly misimplement DNS, because that starts trends, like Heroku suggesting for a long time that you use a CNAME for @ (don't do that).
I'm not making this up: http://i.imgur.com/zFNkV.png