After a bit of checking on WoWWiki's Datacenter list I can confirm that this is a major update to the entire Los Angeles Datacenter. If my glass ball is working correctly I bet we can expect similar extended maintenance periods for the other Datacenters in the coming months before the big 4.0 Patch.
Extended Maintenance 06/08/10 - Select Realms
We will be performing extended maintenance on the realms listed below on Tuesday, June 8th, beginning at 12:01 AM PDT. The maintenance is scheduled for 24 hours as we prepare for the upcoming expansion. These realms will be playable again at approximately 12:01 AM PDT on Wednesday, June 9th. All realms not listed below will undergo scheduled maintenance beginning at 5:00 AM PDT and will be available for play at approximately 11:00 AM PDT.
Aegwynn
Akama
Aman'Thul
Arathor
Azjol-Nerub
Barthilas
Blackrock
Bloodscalp
Bonechewer
Boulderfist
Bronzebeard
Caelestrasz
Chromaggus
Crushridge
Daggerspine
Darkspear
Dath'Remar
Draenor
Dragonblight
Dragonmaw
Draka
Drak'thul
Dreadmaul
Dunemaul
Eitrigg
Eldre'Thalas
Feathermoon
Firetree
Frostmane
Frostmourne
Frostwolf
Garithos
Gundrak
Gurubashi
Hakkar
Jubei'Thos
Khaz Modan
Khaz'goroth
Kil'jaeden
Kilrogg
Korgath
Kul Tiras
Malorne
Mug'thol
Muradin
Nagrand
Nathrezim
Ner'zhul
Perenolde
Proudmoore
Rexxar
Runetotem
Saurfang
Scarlet Crusade
Sen'jin
Shadow Council
Shadowsong
Silver Hand
Silvermoon
Skywall
Smolderthorn
Spirestone
Stonemaul
Stormscale
Suramar
Terenas
Thaurissan
Thorium Brotherhood
Tichondrius
Uldum
Vek'nilash
Windrunner
Thank you for your patience
[ Post edited by Laetzin ]
Laetzin
Blizzard Entertainment (Source)
Links
World of Warcraft - Forums - New extended maintenance this tuesday
WoWWiki - Realms by Datacenter
