I have noticed that in Fedora 8, the /usr/share/gdm/default.conf, the default theme is: GraphicalThemes=FedoraInfinity. If you comment this out and uncomment #GraphicalThemes=circles/:happygnome, the ISPConfig should work using GDM and Gnome.
But there is one thing I DO notice. Installing ISPConfig *does* affect Fedora's GDM. The question is, what exactly, is not known. I suspect it has something to do with changing the passwd, group, and other system files *somewhere* since Fedora uses getent in order for GDM to work with "FedoraInfinity" themes.
I have tried to remove all traces of ISPConfig from my Fedora system and so far I have not been able to restore GDM's original state so that FedoraInfinity theme would work again.