I don't see why any raiding guild would want to do a 25 man instead of just splitting into multiple 10 man groups. The loot quality is the same, it's much easier to manage 10 people/get them together/more people probably get to raid in multiple groups. Even though the loot ratio per person might be slightly better on 25 man, I don't think it's going to be a big enough reason for anyone to want to go for 25 man.
Is this going to be the death of 25 man raids in cataclysm? I don't see ANY logical reason why a guild would do a 25 man instead of 10.