You can now develop for Arduino directly from Android using the ArduinoDroid app, released on Friday onto the Google Play store. The app allows you to develop, compile and upload¹ sketches to your Arduino board directly from your Android phone or tablet. The app is free to download, but displays ads. The ads can be removed with a $2.99 in-app purchase.
¹Sketch upload is only supported for the Arduino Uno and other FTDI based boards. Android devices with USB-host support are required.