The user's manual isn't too detailed about the exact content of the flash settings menu for the 7D, but it still seems that the 7D supports a few more settings when using the built-in flash as a master, compared to the functionality offered by a 580 EX II, when used as a master.
Master flash isn't in group A, but separate, so it can have a ratio vs. flashes in group A as well. For power resons not more power than the external flash, though.
Since internal flash is considered separate, it can have its own flash exposure compensation when in E-TTL II.
So can the A and B group, when using them, not only the C group.
The overall flash exposure compensation is also still there.
When setting the flash power manually, it's possible to set the internal as well as the three external group's power ratio.
Not that I know if these functions do contribute anything real to the images you can make, but it seems that they at least have tried to think outside the ordinary box at Canon.
