When I made that design decision I wasn't considering the possibility of DNS outages at all; I was just thinking in terms of "there's a huge number of places between me and ultradns where someone could insert a spoofed DNS response".

Are there DNS servers that support versioning? The best solution I could imagine would simply be to set Tarsnap to normally use the current DNS records for S3, but be able to rollback to a valid zone record if they encounter an update that makes the servers stop resolving.

you can expect such downtimes every Christmas. why? ask yourself how much protection money UltraDNS was told to pay.

Mostly security -- DNS is one of the worst offenders when it comes to protocols with security problems, both in terms of protocol issues and bugs in implementations.

Amazon doesn't move endpoints very often, and I round-robin requests to several endpoints; so in the rare cases an AWS endpoint changes I see a slight decrease in Tarsnap performance and can make the Tarsnap server stop using the dead endpoint before any users are likely to notice.