Since launch back in 06, the PS3 mobo has changed quite a bit in a years time. It seems Sony is still working on a design that they can be happy with, both production and cost wise.
They are now on their 5th motherboard layout and some will be surprised by the fact that the 40GB white (JPN) and the 40GB black (NA) models are actually completely different.
The history is as follows:
The HDD is installed differently in the White model as well and the back of the unit is different than the Black SKU.
They are perfecting the mobo as much as they can, cutting costs very effectively imo. The cooling method is much different than it used to be on the 60GB model. (pics in the link below)
Click here for more info
I'll refine this if anyone catches any mistakes or has any additions.