[edit: collecting my responses]
My reference is this document – http://www.autodesk.com/techpubs/autocad/acad2000/dxf/ – several of the packages I’m working with read/write INSUNITS to set the unit for the entire document and Rhino also sets the INSUNITS header variable on many of its exports.
The problem is that I’m not seeing it consistently in documents I export. It works correctly enough that I don’t check every DXF export, but from time to time when I load the file in my laser cutter it barfs saying it can’t print something 1280 inches by 500 inches. That file is missing INSUNITS, I edit it in, and can cut just fine.
This is OS X, so Settings->Units and it’s set to mm automatically when I open a new document using the mm template. If I open a new mm template document, make some changes, export the DXF, it usually has INSUNITS set to 4. (note: I’m not certain what happens if I just do cmd-N, maybe some times I get an inches document? I’ll keep an eye on that.)
I have another document (set to mm) that when I export DXF some times has INSUNITS set and other times not. I’ll attach that document, it’s just a test sheet.
L-rotate-180.3dm (80.0 KB)