John Gruber points out that the WWDC invitation this year looks very much like an Apple TV.

I wouldn't disagree - but I would add that it could also be...

an Apple Airport Express router (or an Airport Extreme/Time Capsule, viewed from the top),

...an Apple Watch,

...or a Mac Mini.

Which probably highlights why you shouldn't read too much into WWDC designs - its like an ink-blot test, where you see what you want to see.

(For what its worth, I'm guessing its supposed to be an Apple Watch - if Apple are making a 'homekit hub', I would guess that it would sit inside an Airport router, rather than a TV set top box. What if you have more than one Apple TV in your house? What if you have a big house, and the main TV is out of wifi range of the whole house?)