OK we don't know 100% but here's some information.
Any Windows Phone 7 device. NOPE.
Devices with 4 GB of memory: UNCLEAR.
Lumia's tied to a carrier like the AT&T Lumia 1520 or Verizon Lumia Icon: UNCLEAR - carriers may or may not release it.
In the event it is not released officially, like is all but guaranteed for the AT&T Lumia 920 and 820, the Windows Insider program is for you.
As for the rest, the most recent models (640 XL, 830, 532, etc.) go first. Older models (720, 1020, 625, etc) come later.
That's about it. If you need more information, the search function awaits.