Apple's iBeacon -- whether it's a future product that will work with HomeKit or just a device for use at the company's own retail outlets -- has an official product number: A1573. The sharp-eyed folks at Securifi, who have just started shipping their Almond+ Gigabit Wi-Fi Router and Smart Home Hub, noticed the FCC Application for Certification for Apple's iBeacon.
The testing was performed by Audix Technology (Wujiang) Co. on behalf of Apple. Extrapolating the measurements on the drawing above, the iBeacon appears to be about 5.46" in diameter and powered through a USB connection. There's no word on whether the iBeacon will be available to the general public.
Many thanks to Ram Malasani at Almond for the tip!