First, I work at Zombie, so you can take what I say to heart.
So to clarify.
There are a lot of systems that go into supporting the back end of the game experience. Authentication servers, databases, etc, all make up the back end. Then there are the actual dedicated game servers that you connect to and actually run each match.
The way it works is like this:
All core servers that serve the North American and AUS/NZ player base are located on the US west coast. These servers are like what you connect to when you login, social server (manages clan affiliation, friends, etc) the databases associated with those, and so forth. It was our opinion that an extra 50ms or even 100ms on a friend request was not a significant amount of time and therefore was not critical. In fact, all NA now connects to west.
So yes, when you purchase an item, or add a friend, or create a clan, that request is coordinated through a large pipe that takes it from AUS to the west coast and back.
However, the actual game servers ARE located in AUS (as well as some in West and some in East). So when you go and join a match, you are hooked up with a dedicated match server that is in AUS and is physically close to you. This is critical because in game latency and ping is very important when playing a first person shooter.
At that point, everything that happens in the match is being handled within the AUS/NZ region. At the end of the match it will probably ping the west coast again, but thats about it.
So I hope that breaks it down for you.
The other nice thing about it is, that it unifies our NA and AUS player bases. So you can easily talk, whisper, and friend your buddies on the east coast, and they can easily come play with you (though they will have higher ping when connecting to AUS game servers). So... unified player base, everything working together, but each area (AUS, East, West) all having physical dedicated game servers that are physically close to them.