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32GB is a limitation of iX Haswell, Ivy Bridge could use 64GB - Apple can't do anything about it. Let's hope Intel allows it for Broadwell-K (-Y,-U,-H variants will be restricted to at most 16GB)...

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It's not a Haswell vs. Ivy Bridge distinction, it's a consumer vs. server distinction. Consumer CPUs only have dual-channel memory controllers and Intel's DDR3 controllers can't use 16GB UDIMMs or have more than 2 UDIMMs per channel. The server chips (a handful of which get some features cut off and sold under the i7 brand) have 4 memory controllers and also support RDIMMs and LRDIMMs.

>Intel's DDR3 controllers can't use 16GB UDIMMs

That is a software not a hardware limitation (in particular, I think it is in Intel's MRC). ASUS has firmware with modified MRC code that can support it.

That is in fact incorrect. Consumer desktop Sandy Bridge, Ivy Bridge and Haswell all support 32 GB of RAM via 2 DDR3 channels. Enthusiast desktop processors (Sandy-E, Ivy-E, Haswell-E) support 64 GB, because they feature 4 channel memory controllers.

Once the consumer chipsets move up to DDR4, you will have support for 64 GB of RAM on the desktop. http://en.wikipedia.org/wiki/Skylake_%28microarchitecture%29

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