Well, remember Naxx25? Was cleared in what, 2-3 days after Wrath release?
Four Horsemen - 25th August 2006
1. Death & Taxes, Korgath US
2. Nihilum, Magtheridon EU (EU First - 28th August 2006)
Sapphiron - 2nd September 2006
1. Death & Taxes, Korgath US
3. Nihilum, Magtheridon EU (EU First - 4th September 2006)
Kel'Thuzad - 7th September 2006
1. Nihilum, Magtheridon EU
2. Risen, Alleria US (US First - 8th September 2006)
( sk-gaming.com/content/16397-WoW_world_first_list )
Cant say exacly how accurate the list is, but it does give some look into how it was