Hi there! -- Longtime lurker and arduino hacker, first time poster.
Wanted to let fellow arduino folks know about a small open source project of mine, GitHub - mik3y/usb-serial-for-android: Android USB host serial driver library for CDC, FTDI, Arduino and other devices..
usb-serial-for-android is a library that you include in your Android app in order to talk serial devices. It works, without root, on most Android 3.1+ devices that have USB Host support. Just connect your arduino to your android with a USB OTG cable, and away you go. There is software support for FTDI and CDC serial drivers, meaning you can use it to talk to most Arduinos.
If you're considering adding a display, bluetooth, ethernet, or other capabilities to your arduino project, usb-serial-for-android might open up some new opportunities (just connect your arduino to spare/cheap android device instead). I know it's given me all sorts of new ideas..
Please feel free to give it a spin; there's a discussion list linked on the project page.
cheers,
mike