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Here's a recent thread from the OpenStack mailing list. The second message describes the theory behind how to do this:

Basically, Keystone is an HTTP service, so you can make it distributed and highly available just like any other web site. The key is to make sure that the data that Keystone relies on is configured in a distributed fashion. There's no set way on how to do this, but the OpenStack HA Guide might give you some pointers: