One of the CLS (Nikon’s Creative Lighting System) drawbacks faced by Nikon D70 owners was that when the popup flash was used to trigger a remote SB-600 / SB-800 flash, the popup flash itself couldn’t contribute any illumination to the scene.
Well, it’s good that starting with the Nikon D80, this disadvantage no longer exists.
Not only can the popup flash contribute to the illumination of the scene, you even get to adjust the precise amount of exposure compensation to your liking.
Miancu contributed the following test images that highlight this feature on the D80. Note that the remote SB-800 flash is illuminating the soft toy from the side, whereas the light from the popup flash illuminates the soft toy from the front (in the right thumbnail).
View a larger version of these images (warning: long download time).
You can read up more on this feature on Page 108 of the Nikon D80 Manual.
Essentially, you select the appropriate value for the built-in flash, which is either — (popup flash does NOT contribute any illumination to the scene) or TTL (popup flash DOES contribute illumination to the scene, in addition to acting as a commander for the remote flash), in the Custom Settings (CSM) menu, option 22 (Commander Mode).
Nikon Speedlight / Flashes / Lighting Tips — Main page.