alright everyone. it seems there's only one way to run linux 3d apps under freebsd: you need to switch back to linux kernel version 2.4 emulation. the linuxulator is not fully compatible with the linux kernel version 2.6 yet.
here are the steps necessary:
1. uninstall the linux_base-* port installed on your system
2. do `sysctl compat.linux.osrelease=2.4.2`
3. do `echo "compat.linux.osrelease=2.4.2" > /etc/sysctl.conf`
4. do `echo "OVERRIDE_LINUX_BASE_PORT=fc4" > /etc/make.conf`
5. install emulators/linux_base-fc4
rebooting or reloading the linux kernel module isn't necessary! now you should be able to run most linux 3d apps. i tested the quake 4 demo and the ut2004 demo so far.
if you update or reinstall the nvidia drivers be sure to execute the following script afterwards in order for id software games to find some necessary binaries:
ln -is ../../lib/libGL.so.1
ln -is ../../lib/libGLcore.so.1
ln -is ../../lib/libnvidia-tls.so.1