Kraken's just started USD as well, though I don't know (don't have an account) whether ETH/USD trading is possible yet.
I thought they'd always done USD in a limited manner and only in a small number of states, but EUR was their preference. They've made plenty of acquisitions recently so I guess they'll be opening up in a big way soon.
Ah, my mistake. It sounds like this is the "opening up in a big way" ("Update: Launched in the USA, free USD & CAD deposits"), following the Coinsetter acquisition, but due to a (new?)
relationship with SynapsePay.