Never mind my original post. Spoke too soon.
See this document from SRAM on adapter configurations:
http://cdn.sram.com/cdn/farfuture/up...r_mounting.pdf
And also have a look at this Avid Post Mount Brake Adapter kit:
Avid Post Mount Brake Adapter | Competitive Cyclist
It appears that a 160mm rotor shouldn't need an adapter.
I do have an adapter on my Salsa Fargo's post mount fork, but I'm running what looks to be a 180 mm rotor.