Well, it had a notice that the domain service was having issues at the time I posted it. Looks like it's been resolved in the meanwhile.

Near as we could tell from the outside, ns39.domaincontrol.com and ns40.domaincontrol.com were both down. We had a domain that was on exactly those two name servers, so wouldn't resolve at all. Figured that it might be a useful heads-up for anybody seeing any DNS weirdness.