I have just come across this problem with a source code install. The way the system was build was using make DESTDIR=/arch/pkg/32/gnome-terminal-2.28.0 install etc for each gnome package. The contents of these directories was then copied onto a clean file system before being run under KVM.
Running gnome-terminal produced the app->default_profile_id != NULL error. I found the problem was to do with the fact that while the schema files were copied correctly into /etc/gconf/schemas these were not been properly installed into gconf.
GCONF_CONFIG_SOURCE=xml:merged:/etc/gconf/gconf.xml.defaults /usr/bin/gconftool-2 --make-install-rule /etc/gconf/schemas/desktop_gnome_interface.schemas
GCONF_CONFIG_SOURCE=xml:merged:/etc/gconf/gconf.xml.defaults /usr/bin/gconftool-2 --make-install-rule /etc/gconf/schemas/gnome-terminals.schemas
put the necessary entries into gconf for gnome-terminal to work correctly.
All source files are from GNOME 2.28.1. A quick test using
gconftool-2 -R /apps/gnome-terminal
should show if the profiles are installed in gconf correctly.