Death and Taxes has been hard at work racking up those world first kills, but they've encountered a problem with Kel'Thuzad - namely, the event begins, ends, and there's no Kel'Thuzad to fight. Nnep describes the problem thusly:
...during our buff period we noticed he sort of, uh, dissapeared. Phase one still triggered as normal, although afterwards when kel'thuzad would normally become an entity, a la thaddius, he was still missing.
Not that you can blame poor Kel'Thuzad - several forum posters have speculated that if they were a raid boss and saw Death and Taxes running down the hallway, they'd disappear as well. Fortunately, however, Tigole claims this is not the way the encounter is intended - and perhaps we'll be seeing another world first soon.