In the end it came down to the fact that I was using the wordpress 3.0.5 Debian sid package. According to the Debian guidelines, Debian packages should not ship software that has already been packaged for Debian. This is why the wordpress package depends on some PHP libraries and also tinyMCE and symlinks to them from inside the /usr/share/wordpress directory. Apparently there were some incompatibilities between some external libraries and wordpress 3.0.5 which prevented me from inserting objects into the editor.
Long story short, I’ve removed the Debian package and am now running wordpress from source, which works like a charm.