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".
Ah, my bad, sorry I thought you meant in the tarsnap client. Then I really don't know why people would make a fuss over you hardcoding the ips in there. All you need to do is keep an eye out in case they change them (which you could even automate).
