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.dxf is open , blender is gpl and can import & export to dxf
DXF is a text format which means it is much simpler to understand than a binary format (such as DWG) but this does not mean it is open.
Even though, it still requires a lot of work to translate what is in the file to the object model that Archimedes use.
This is part of the top features missing but I still can't predict when I will find time to implement it. If you do find a developer willing to spend some work on it, I am free to help with Archimedes' part.
I've added the following references from other posts.
http://sourceforge.net/projects/ycad/ - A library to read and write dxf files. A bit outdated (last update was in 2003). - LGPL
http://www.qcad.org/ - It is in C++ but they offer a dxflib - Dual license (GPL and a commercial one - GPL is incompatible with EPL and just looking at GPL work may taint the whole outcome as a GPL derived work causing some licensing issues.)
One more reference:
Open source GIS program in java. It import DXF, DWG, and DGN