Do you have libGL installed? If so deinstall that port. LibGL and the nvidia-driver are mutually exclusive. They both do the same thing, X might be picking the libGL one instead of the nvidia one. Look at the logfile. I've attached mine so you can compare. GLcore should not be loaded. Only libglx and nvidia_drv should be loaded.
How did you install Xorg 7 btw? Did you use the instructions at http://wiki.freebsd.org/ModularXorg
or did you do it by hand?