Faber Acoustical is coming to your rescue with their US$19.99 SoundMeter for iPhone. They discovered that both the built-in and headset microphones of the iPhone have a flat enough frequency response to make them useful for measuring sound levels.
To quote the technical part of their press release, "SoundMeter measures average sound levels with fast, slow, and impulse time weightings. Peak, maximum, and equivalent sound levels are also available. All sound levels may be measured with Flat, A, or C frequency weighting." Got that?
Faber notes that SoundMeter doesn't meet ANSI or IEC standards for precision sound level meters, but it works very well as an inexpensive level meter for hobbyists. They recommend not using the built-in mic on first-generation iPhones (the headset may be used instead), and SoundMeter should be calibrated prior to use.
Faber Acoustical also produces two other really impressive iPhone apps, SignalScope and SignalSuite. SoundMeter for iPhone is available in the App Store now (click opens iTunes).