iFixit has a teardown of the PlayStation 3 Slim. What did iFixit find? That the Blu-ray optical drive still takes up an assload of space.
Sony can shrink down the other parts, but it's hard to shrink something that needs to work with a fixed size component (the Blu-ray discs). However, the logic board and everything else does seem smaller, maybe to make room for that massive fan. [iFixit and Rapid Repair]
Update: Rapid Repair's doesn't seem to be up yet for some reason.