The fan is literally always on if the machine is on. It may not be audible but it is on and in fact providing cooling to the CPU.
Broadwell Y series SKUs will have the capability to be fanless but will come at roughly a 35% performace decrease (using current Haswell comparisions) to the U series counterpart:
4250U - 3479
4210Y - 2247
4650U - 4357
4610Y - 3514
Note that the Y series SKUs also use HD 4200 graphics, which are much worse than HD 5000. They are roughly equal to HD 4000 graphics from Ivy Bridge.
The Y series SKUs are also expected to see a roughly 30% power drop but at the same performance, there has not been any mention of Broadwell performance increases as of yet.
Also, Broadwell mobile so far does not have DDR4 support confirmed, only Broadwell desktop and Haswell E have been confirmed.
So if the machine does go Broadwell Y its looking like it will have better power consumption, much worse graphics, the power of the current i5 processor, the same RAM and undoubtedly worse performance when combined with a retina display...all for the same price!
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I've said this before, but when you look at the positioning of the MBA its too good right now. Performance wise its only about 8-10% away from the 13" rMBP and is cheaper with better battery life and portability.
Take into account the BOM cost of the MBA (estimated at $718 back in 2010 when a Core2 Duo was only $80 vs $300+ for a current i5) and the product line has reached an inflection point.
Personally I'm hoping Apple goes to a lower cost ARM route and, in effect, starts some profound changes in the mobile computing landscape.
DDR4 memory is so perfect for the rumored new ultra-thin, fan-less 12" MBA that I can't see Apple announcing it without it.
1) DDR4 can use much less power.
2) DDR4 is much faster.
Lower power is a key to making the laptop fan-less. Faster subsystems (like memory and SSD) are key to making up for the lower processing performance of a fan-less Broadwell part.
I don't see Apple releasing it without Broadwell that supports DDR4. I think it will have both.
It would be a joke if it had an ARM processor...