Apple's big reason for using the 27 inch was for securing a forfactor to fit in those high tdp lynnfield chips in the high end iMacs. About the only thing Apple could do with a 30 inch iMac would be the probability of fitting in SLI'd/crossfire'd mobility gpus or a desktop gpu. Seeing as how Apple doesn't focus on graphics performance, but rather cpu performance, I see no need for them to up the screen size again (especially since every new gpu/cpu with each release has a smaller manufacturing process and thus less heat produced)