We're using HyperFilter, at least for beta, so that's Amsterdam, NL. They should scale up enough to meet any attacks (it doesn't look like much, but a few of the larger servers are using them and things are kept under control fine), but we have Incloudibly (Zurich, Switzerland) lined up for official (or even beta, if we are forced to scale beyond HyperFilter's limits -- which we shouldn't).
We'd be using Incloudibly off-the-bat for beta (as was the plan), but they're rather expensive (but so worth it!) -- and we've dragged alpha out so long that we may not be able to afford paying it out of pocket for too long.
We'll certainly be switching over to them for official though.
Edit:
Latency wise I wouldn't be too worried regardless of where it's hosted. I've been told by several people that the server itself feels so much smoother (because of the way we handle various things), despite (them) having a ping of 350ms+ (to our dev or alpha servers -- not representative of beta), so at least on that front everything should in general feel a lot better.
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