They are not shipping 24-inch iMacs until Tuesday, perhaps because there will be a rev in the 24 on Tuesday. The 24 has a bigger chassis than the 17 or 20, even if made slimmer, so it is the best candidate to use hotter (non-mobile) cpu and gpu and to sport a new design and maybe aluminum case.
It doesn't make sense for apple to release the 24 inch before the rest of the line, and before the ilife revision.