The list of realms that will have 24 hour+ maintenance downtime is here. No word on how long other realms will be down.
http://www.wow.com/2010/07/19/24-hour-maintenance-for-select-servers-on-tuesday/#continued
UPDATE; All realms apparently are still down until 6 p.m. PDT due to problems with the authenticator server. Can't log in until this is fixed.
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