Blizzard have made this announcement regarding upcoimg mainteance by one of their Internet Service Providers.

One of our internet service providers will be performing maintenance on Thursday, June 29th beginning at 10:00 PM PDT and lasting until 2:00 AM PDT Friday morning. This maintenance should not affect any play experience during that time. The following realms are located on the World of Warcraft site where the maintenance is taking place:

Cenarion Circle
Cenarius
Hyjal
Lightbringer
Uther