As long as Eth can't be exchanged directly for USD, it's not possible for it to take btc over.
You can do EUR on Kraken. And any exchange operator with dollar access and half a brain will find those volumes pretty hard to resist.
Kraken's just started USD as well, though I don't know (don't have an account) whether ETH/USD trading is possible yet.
It is. Last 24 volumes were: