Hi,
I would like to share my plugin ...
Just copy the dxf_export.rb file in your plugins folder.
You'll find it in 'Tools->Export to DXF'
It outputs polygons|polylines|cirles|arcs|lines to DXF (contours only)
(2d orientation for circles and arcs 
these do not have a 3d one but the centerpoint is positioned in 3d space)
3d rotated circles and arcs will be output segmented (lines)
polygons, polylines, lines, 3d circles, 3d arcs as lines
2d cirles, 2d arcs as circles and arcs (NOT lines) !!!
I did this because I use the DXF with heekscnc to generate milling operations (G-Code).
Milling a circle or an arc is much faster than several lines ...
I had some trouble with the rotation of the arcs but I think I did it 
History:
*** v0.0.4 (2011-10-28)
- fixed arc orientation (finally?)
- model won't be exploded anymore
- 3d rotated circles & arcs export to lines
*** v0.0.3 (2011-10-22)
- fixed arc orientation
- improved objects assignment ( faster)
- corrected calculations ( x/2 returned int and not float as needed
) - automatic unit detection (to be tested
)
Any arc orientation testers online? 
Comments still welcome!
Jörg