olivier: It's not possible for Airfoil Speakers to simply "declare itself as an AirPlay compatible receiver". This isn't really a "point of view", it's reality. Again, Apple's software (iTunes and iOS) do not acknowledge anything but their approved hardware.

Licensing directly from Apple is not currently possible for software. AirPlay licensing is only available for hardware devices, and requires a hardware chip. It appears that Denon's "software update" simply activated hardware which had already shipped in the device.