I was reading some item (on HN probably) about how they have a big one in Tampa, Florida, a small one in Amsterdam and are planning on building another big one in Virginia.
That said, the article mentioned a time when a failure in Amsterdam lead to a failure to fall over to Florida which took out the whole service for a good while.
I live in Tampa. We are the Lightning Capitol of North America {insert Wikipedia link here}. Right now is the rainy season here, and generous amounts of lightning. It's basically the monsoon season. So the power sucks here, and even if you have excellent generators, as Home Shopping Network does, the network connectivity is still bound by local limitations. Tampa is a terrible place to put a data center.
I think it's there for historical reasons, because Jimmy Wales used to live there, so his company was there, and now it's a significant project to move everything.
According to this, it looks like all the db servers are in the Tampa datacenter, with Amsterdam (and formerly Seoul) just having miscellaneous utility servers, backups, and a bunch of Squid proxies: http://wikitech.wikimedia.org/view/Server_roles
Looks like way too few servers in the Amsterdam cluster to fail over to anyway, even if they were set up for it: the Tampa datacenter has over 400 servers, versus Amsterdam's 44.
That said, the article mentioned a time when a failure in Amsterdam lead to a failure to fall over to Florida which took out the whole service for a good while.