We all know the iPhone 4 has a 1GHz A4 processor, but how much of that 1GHz are we using? I think we've all come to the conclusion that it is under clocked, but to what?
It's not like 'Angry Birds' says "recomend 600 mhz or better iPhone" in the app store or anything like that.
Developers test different phones and then say which iPhone model and iOS version they require for their app to run. That's all you need to know. There's no reason for them to say more than that and no reason for me to care.
Benchmarks show that the iPhone 4 runs integer code loops at 80% the speed of an iPad, which runs the same A4 processor at 1 GHz. So a very good guess would be that the i4 runs the processor clock at 800MHz.
Planetbeing suspects that the iPhone 4's clock speed is dynamically scaled. That makes sense -- that would give the best balance of speed and power consumption.