Ongoing
My domain registrar's authoritative nameservers are down. This is ultimately out of my hands; it's no fault of my own and there's nothing I can do to expedite the fix. Anything I do to bypass my domain registrar's authoritative nameservers will only propagate globally in approximately 48 hours, by which my registrar will almost certainly have fixed it. By now, someone's pager is probably alerting them on a nice Sunday afternoon. Hoping for a speedy recovery.
Some clients may not experience any outage: using the service regularly means your router, DNS server, etc have lots of chances to cache the DNS response. With a TTL of 5 minutes, this might not last long. I'm not sure the specifications regarding returning the last known IP in case of an error.
Nonetheless this is an unsatisfying blow to my uptime (especially because it's not my fault). Rest assured I responded within minutes to diagnose the problem. Now, though, my hands are tied, and I apologize.
Aftermath
The domain registrar contacted me to let me know that there was a DDoS attack on their DNS servers.