Several recent posts have dealt with the topic of interfacing Arduino with devices using the Android operating system. Here’s another approach: Amarino. “Android meets Arduino” is a toolkit, basically consisting of an Android application and an Arduino library which will help you to interface with your phone in a new dimension. You can build your own interfaces with little programming experience.
If you want to work with Amarino you need at a minimum an Android phone (Android 2.x works best, however Android 1.6 is also supported to some extent), an Arduino board and a Bluetooth shield for your Arduino. Download the software including the MeetAndroid library and the companion Android app.