Tellart’s NADA Mobile was launched a few months ago. Since then, the Tellart folks have released the source code under an open source license, so you can use it to build your own iPhone applications using Apple’s Dashcode as your development environment:
NADA Mobile is an enhanced version of Mobile Safari that gives you access to the accelerometer, GPS, microphone–and can even be used to read the value of an external analog sensor attached to the device’s mic input (without using a computer or a microcontroller).
I love the way they’ve integrated sensors into NADA Mobile. Like the external keyboard solution shown off by Perceptive Development in iPhone Hacks, Tellart uses the microphone in port. But instead of working with serial connections, Tellart’s 1/8-inch Jack uses a simple design to read the value of variable resistors. Check out the instructions for talking to a sensor from NADA Mobile: Tutorial: Creating an Application Sketch w/ Sensor