firing a 580ex wirelessly with a 7D

I think it still fires the built-in flash but that flash doesn't effect the exposure of the image or show up in the photograph as the power output is so low.
 
As above.

The flash will still have to transmit the E-TTL data, but once it's sent the trigger command it dies, therefore supposedly not adding to the exposure.

It will look as though it's popping off, however!
 
Yes the falsh you see from the built in flash is merely a pre-flash to gain the correct flash exposure for the image, this information is then transmitted to the other flashguns.

All in a matter of milliseconds
 
If it's anything like an external flash gun, even when the pop-up is set for pre-flash only and the main flash is disabled, it still fires, but at a very low level. You can see it reflected in mirrors and stuff, but the power output is so low that it doesn't affect the exposure unless you are very close.
 
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