I don't understand the thinking. Is it to reduce costs?
Surely, just dropping a few GB on a HDD doesn't drastically reduce costs?
Far from it, I know that as time moves on, everything increases - be it sizes of HDDs, or consumer demand for space.
Therefore, what may be cheap to source today (40GB 2.5in HDD), could end up costing twice as much as a 80GB drive in a year (or two), as comsumer demand for the smaller drives drops (in the marketplace), and market forces start to drive prices up.
I suppose I should just be greatful that SCE aren't making the HDD optional, and telling people to buy SCE-branded USB memory keys at £20 for 128MB (á la £20 for a 8MB PS2 card)!!