Random thoughts:
How about the cost of the chassis? By that I refer to the PS3's seemingly complex design, just by looking at it. The Xbox 360 on the other hand, is pretty much...well... a box.
Its just all injection molded plastic pieces isn't it?
And the built-in wireless of the PS3 must cost something significant too, perhaps not on the same scale as the processors, but it is the difference between including one and not including one.
Prolly in the range of $5 or less considering I've bought Laptop miniPCI wireless NIC's for $10. (edit I mean you can consider that even the DS has wireless built in which always makes you wonder why for $20 more than the Microsoft Wireless Dongle you can get a DS)