i think it would be cheeper overall to release dual dual-core proccesors, as apple wouldn't have to re-design the motherboard and stuff like that.
if they can simply slot in 2 dual-core proccessors in place of the orignal ones it'll make their life alot easier, it'll also see the powerPC chips out in style, rather than 'upgrading' the powermac, even tho it won't actualy increase the performance.
no-one realy cares if they're powermac runs abit cooler, the case size will be the same, people want an apple computer that can compeat with the speeds of PCs.