Intel was attempting to force users wanting to use their newest Devils canyon unlocked K series processors to upgrade their motherboards to Intel's Z97 chipset motherboards. This did not sit well with alot of users who had just upgraded to a z87 motherboard. Asus has been a bit rebellious as of late as they recently chose to ignore an Intel NDA regarding their Z97 series.
Well it looks like they are at it again. Most, if not all of their Z87 boards have received a bios update allowing the use of Intel's devil canyon processors. This is also backed up on their Cpu support lists.

MAXIMUS VI IMPACT BIOS 1504
Enhance compatibility for new Anniversary Edition and Devil's Canyon CPU.
Note: Must apply the attached BIOS updater tool first before using the new Intel 4th Gen Core processors