Although the built-in wireless flash trigger in the 7D is a very handy feature, it seems it can't expose properly.
Take a look at these two images. They are both shot with the master flash turned off and with two 580 EX II slaves, A and B, each illuminating the subject indirectly from the left and right.
In these two images, one is shot with the internal flash as a master, the other with yet another 580 EX II on the camera, acting as a non-contributing master.
Flash power is in both cases set manually (1/8 ) for both A and B.
The camera is in M mode, so shutter speed and aperture doesn't change. One could expect that the two images to be virtually identical, right? And indeed they are. There is hardly any point in telling which uses internal master and which uses external, since they are both OK.