The PS2 re-design was about reducing an already ridiculous cost efficient system to further make way for cutting price. It basically sealed the deal as the number 1 system. You can cut internal costs as much as you want, but eventually the redesigning the shell becomes the only way to reduce costs further.
The PS3 re-design was a necessity. The only way for SONY to reduce prices more without breaking the budget (meaning the hardware division, not necessarily the parent company) was to do the SLIM. I expect a year from now or so for SONY to further reduce costs of the internals. Eventually near end cycle we may see another re-disign of the shell.
Lets not forget the internals for the PS3 and X360 have already been refreshed at least once. We are sure to get more.
I dunno the cost breakdown of the X360 casing but after one or two more internal refreshes MS might be forced to redo the casing if it wants to further reduce costs.











