Looks like the test realms ae open again. Here's what Bilzzard had to say about it:

Once again, the Public Test Realms have opened their doors, allowing players to investigate and help test the upcoming changes in patch 1.6. This patch contains a plethora of class changes (particularly to Warlocks and Warriors), the new Battlemasters, and much more!
Players can participate in the test realms by creating a character or by copying their characters from existing realms to the test realms. Those who have previously copied their characters to the test realms will need to do so again, as all previous characters have been removed from the test realms. To copy your character and to download the test client, visit our Test Realm Character Copy page.

Please be aware that the test realms allow us to conduct a broad test of major content patches, and based on the results of the public test, further changes may be introduced before a patch goes live. To read more about the test realms, please review the Test Realm FAQ. We truly appreciate our players taking the time to help us test the latest changes to the game; thank you all!