a) export with a black rectangle empty:
You need to convert rectangles to paths before exporting to DXF else they do not get exported.
Reproduced with Inkscape 0.48 and 0.48+devel r9817 on OS X 10.5.8.
b) import of externally created DXF files empty:
Can you attach a DXF file created by another CAD application that doesn't import into Inkscape?
a) export with a black rectangle empty:
You need to convert rectangles to paths before exporting to DXF else they do not get exported.
Reproduced with Inkscape 0.48 and 0.48+devel r9817 on OS X 10.5.8.
b) import of externally created DXF files empty:
Can you attach a DXF file created by another CAD application that doesn't import into Inkscape?