Idle power is hugely important for mobile. I don't think though there will be a 3W difference for the mobile parts - but GF108 supports Optimus which could be just what the OEMs want (yes I'm aware that's pretty much a difference in software only, but the OEMs won't care if that's hw or sw...).That's the point, a quick rebrand is cheap to do and not all sold laptops will have SB's on board.
104mm^2 vs. 130mm^2? With the HD5570 @ 39W being faster than a GT430 @49W? At least it idles 3W or so lower...
It's true for desktop graphic cards the HD5570 walks all over GT430 while drawing less power under load, though I'm not sure the situation is the same for the mobile parts (could be closer there depending on clocks - something hard to judge given those parts can ship with vastly different specs).