ISpectrum - Macintosh Audio Spectrum Analyzer. Use audio spectrum analyzer that allows you to view live audio in a standard frequency plot. Online Manual.
Synth 76477 is a powerful sound effects generator and synthesizer app, inspired by the famous SN76477 integrated circuit from the 1970s, but containing many enhancements.
Wirelessly posted (Mozilla/5.0 (iPhone; U; CPU iPhone OS 41 like Mac OS X; en-us) AppleWebKit/532.9 (KHTML, like Gecko) Version/4.0.5 Mobile/8B117 Safari/6531.22.7) There are some videos on YouTube that play tones (mostly focused on bass tones) at set frequencies, with the frequency displayed on the screen. While not exactly scientific, it could give you a quick and dirty answer. I used them to test out the factory sub/amp in my wife's vehicle. Edit to Add: Unless you're including the earpiece at the top, the iPhone only has one speaker. If you need more information than the GMS Arena review, you might want check out. This developer offers an app (Audiotools) that is meant to analyse the characteristics of sound with a matching calibrated microphone (Audiointerface).
The app/microphone combination offers capabilities for characterizing sounds that would cost much more if you used dedicated sound analysis hardware. If you are developing an application that mimics the sound capabilities of the iPhone4 (great idea by the way since using the EQ on the iPhone/iPod chews up so much battery power), you might want to invest in the system since you'll have to adapt it for each new model of iPhone. I am not affiliated with the company, but I do use the app & microphone in my psychological laboratory to make quantitative measurements of sound frequency and intensity and I am very impressed with it. I use the system with the cheapest iPod Touch and the combined cost the app, microphone and iPod is about 10 times less that a dedicated sound meter with similar capabilities. There are even add-in modules for the app that would do precisely what you want.
I'd take the measurements for you and post them here, but alas I am a poorly paid UK scientist and can only afford a 3GS.