If some of you actually try Dia - a simple software for creating diagrams, a visio alternative (a very simplistic one) and after all the work finds that the exported .png file has very low quality this is how to fix it:

It seems that when exporting with Dia if you do:
Under "File->Export..." in the dialog select: "Determine file type: By
extension" and under "Name: " put something like "test.png"
the final .png has poor quality, and no antialiasing is used.

But by doing:
under "File->Export..." in the
dialog select "Determine file type: Portable Network Graphics (*.png)" and
Under "Name: " put something like "test2.png"
Now a window will ask about image size, just click "ok".
the final .png now is antialiased and the quality is perfect!

I described the bug here with a simple example: http://bugzilla.gnome.org/show_bug.cgi?id=535179

If someone can reproduce this I would appreciate to comment on the bug. Thank you.

I actually lost several hours trying to fix this so I hope this will be useful in saving your time...