The GNOME thumbnailing stuff seems to be wedged inside libgnomeui -- which is deprecated. But where did this move to? GIO might be a reasonable place, but it's not there (yet?). Perhaps the Hildon thumbnailer might be a better choice, as it seems to have fewer dependencies. (hildon-thumbnail in Debian, sources in SVN) -- or even the Thunar thumbnailer? The benefit is that (because of the fd.o spec) these are all interoperable, so it's easy to change later.
It would be nice if there were nice python bindings, but it seems I'd have to roll my own in any case. (Maybe PyBank would help?) Does anyone have any thumbnailers I've missed, know where thumbnailing is expected to live in the GNOME/GTK stack, or have any other helpful advice?