Besides, just how many phones do you think are needed to even generate 1 GH/s?
I would say an iPhone 5 probably can do between 10-20MH/s, so if the got a million of those hooked up and mining, it would be about about 10-20 TH/s
That's a tall order though...
That assumes a million online at all times. Likely each of those will only be online for a few hours a day max. Anyone on an iPhone would need to run the app, it wouldn't run in the background for very long at all.