First DNSimple and now Zerigo. I wonder if this is coordinated or if it's separate groups. In any case I'm glad I switched to Route53 a few weeks ago.
Almost three years ago, DnsMadeEasy (which as far as I know dwarf DNSimple and Zerigo) had an outage after a massive attack.
"Zerigo has procedured DDOS mitigation services from Verisign and is in the process of integrating them into our network to prevent such attacks from compromising our DNS services in the future."