At least 3 of the 13 global root servers were briefly overwhelmed in a sustained attack last night, lasting, in some cases, as long as 12 hours. Looking at the RIPE NCC DNS monitoring service it seems that 2 of them, G (US Dept of Defense) and L (ICANN) were having the most difficulty dealing with and recovering from the attack.
At this point the attacks seem to have been aimed at UltraDNS, which primarily handles traffic for .org sites.
While there is as yet no mention of who is suspected or what their motives may be, there was a similar attack last year on UltraDNS.
