“Is DWF an ‘open’ file format?”

Yes, consider the following:

Published Spec

Published Toolkit source code

Sample code

Standards based

ZIP/ZLIB

XML

OpenHSF

JPG/PNG/bitonal-G4-TIFF

In addition:

Free Windows Printer driver "Autodesk DWFWriter" to produce DWF from any Windows application.

Free Windows Autodesk DWF Viewer.

For more information please see:
http://www.autodesk.com/dwf
http://www.autodesk.com/dwfit

DWF v6.0 Toolkit
The DWF™ 6.0 Toolkit allows you to develop applications that read or write multi-sheet drawings in DWF™ (Design Web Format™) format.
Autodesk DWF Viewer
With Autodesk DWF Viewer, a small, free, downloadable application, anyone on your team can view and print DWF files.
Autodesk DWF Composer
Autodesk DWF Composer supports complete digital round-tripping of markups, annotations, and other changes back into the AutoCAD® 2005 family of software products.

Cheers,
-Shaan