Yep it's me again :-) After some time I spent searching around for other possibilities to write IPTC fields to JPEG files I found this one:
http://www.zonageek.com/software/php/jpeg/index.php
This one seems to work pretty fine, remember that you have to install PEAR to get this running (Debian package php4-pear).