Export had some things wrong for years, like:
 - images were flipped upside down
 - WPG Polycurve was not used due to some misunderstanding with the
   specification http://www.fileformat.info/format/wpg/egff.htm
 - object positioning was slightly wrong

Import was only test code compiled on Windows, now:
 - split test code to it's own file and make it a full WPG importer
 - can read in again everything Dia writes with the exporter
 - some type information still lost due limitations in WPG (e.g. font
   handling, image bit depth and alpha) as well as limitations with
   Dia (e.g. no filled arc object yet)

Exported files (render-test.dia, std-props-test.dia bezier-approx.dia)
were tested for conformance with WordPerfect 5.1 (DOS) and "My ViewPad"
4.1.122 (Windows 7/32).
Also some test were performed with Inkscape 0.48.1 but Inkscape's WPG
import has issues with positioning, filling, line style, text, arcs and
poly curves at least.