I have been on a quest to figure out how to do pcb milling using the gEDA suite of pcb CAD software. My goal has been to go from schematic to milled pcb with free and open source software, including the operating system - I use Ubuntu/Linux. So far, I have found a way to go from "schematic diagram" to ".dxf file" using the gEDA suite, Inkscape scalable vector graphics editor, and pstoedit.
I also have found a way to go from "gerber file" to ".dxf file" using the same set of software tools. So, from any pcb CAD software that can produce gerber files, you can make a .dxf outline of all the copper traces, pads, and polygons (ground planes).
Here is the link to the thread I started on this:
Schematic Capture to dxf File - using gEDA, Inkscape, and pstoedit