When you use the pop-up flash on the 7D as a ETTL Master, you have the choice of whether the flash is used as a light source or not. If you choose to not use it, it will not fire when the shutter is open. Don't mistake the pre-flashes that are used to communicate with slave flashes, with the actual exposure flash.

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Edit and Update:

Based on the request from @Imre, I will update my answer as best I can. Unfortunately, I do not have immediate access to a 7D, as my experience is with another photographer's camera. Therefore I will post evidence of others, until I can produce my own.

My first link is a shooter on FredMiranda.com forums, who posts two pictures, one showing no flash contribution from the 7D pop-up:

http://forums.dpreview.com/forums/read.asp?forum=1019&message=33209810

Further down this same thread, user "Jehuty" reports that they configured the 7D for wireless control, and turned off the external flashes, taking a photo of the 7D in a mirror. The result was no visible flash. Unfortunately, there is no photo evidence of this provided.

Finally, there is information on CPN (Page 2, "EOS 7D: Integrated Speedlite Transmitter uses") that discusses this tangentially. It suggests that you can use the pop up flash as a fourth group, but that its power will be reduced to a GN of 4, due to the power of the flash being first used to signal the other flashes. As Canon suggests, this has little impact other than providing catch lights in eyes.

EOS 7D: Integrated Speedlite Transmitter uses

Thanks to its Integrated Speedlite Transmitter the built-in flash on the EOS 7D can be used as the master in a multi-flash set-up to control wireless slave units, but it can also be used as a fourth lighting group to add a little bit of front light. If you do this though, the power from the built-in flash will only have a maximum guide number of 4 because it will have used the majority of its stored power to trigger the other remote flashguns. However, this may be just enough to add a gentle catch light to a subject’s eyes without a high risk of causing redeye.

Though they do not say it explicitly, this again suggests that unless you enable it as a fourth group, the pop-up flash does not fire and contribute to the image.

Until I, or someone else can contribute a set of images taken in a mirror, I suspect this will always be a contentious issue. However, the 7D manual and CPN both suggest that the flash does not fire at the moment of the shutter being open.

I do not what to make a scene of this, and am happy to be proven wrong, in which case I will gladly withdraw or update my answer, as it would not be contributing to the knowledge of this board. I am not here to be 'right' I am just doing my best to add to the knowledge. Unfortunately, I am not able to test and confirm, and hope that others can to 'illuminate' this topic.