The rasterization that happens in your example file is predictable and
needed, because a lot of the objects used a minimally reduced object
opacity (96%) - seemingly either to mimic a lighter shade of the fill
color, or unintentionally, as the visual effect is barely noticeable
compared to using 100% opacity.

Indeed this was unintentional - I must have goofed it up at some point. Removing the transparency solves all the issues with this picture.

The next cairo version (1.12) will again support the psfrag method to
extract text from the EPS file (limited to latin characters, see comment
#43).

that's good news.

thanks for you help! best, Michael

~suv

[1]
<http://wiki.inkscape.org/wiki/index.php/Release_notes/0.47#PDF.2C_PostScript.2C_and_EPS_export>
[2] described e.g. in these comments from a cairo developer:
<https://bugs.launchpad.net/inkscape/+bug/336638/comments/15>
<https://bugs.launchpad.net/inkscape/+bug/336638/comments/17>