I went to O'Reilly's "Android Open" conference in San Francisco this week. I was not enthusiastic about it, thinking it would all be about app development, but I got a 100% off discount code so I went. In fact there was a heavy hardware emphasis, especially in relation to Arduino. Google has adopted Arduino as the platform for accessory development for Android. .
For devices with Android version 2.3.4 or later it is very easy to use Arduino to develop accessories that connect to the Android device via USB for Arduino boards that support the ADK (there are at least two out already, and Sparkfun showed a third that isn't out yet) , . If you're getting into Arduino, buying one of the boards that supports the ADK might be a good idea, they are only a little more expensive than the non-ADK boards.
It's a bit strange, but since few Android devices have host USB ports (some tablets have them, but no phones that I know of), the accessory is actually the host and the Android device is the target. This means that the accessory also needs to provide 5V, 500mA to the device (which I thought strange since there is an option for a host device to provide less current).
The founder of Arduino, Massimo Banzi, spoke and was giving away and signing his _Getting Started with Arduino_ book. Simon Monk, author of _30 Arduino Projects for the Evil Genius_ was there so I brought the book on the 2nd day and had him sign it. There was also a presentation entitled "Arduino & Handbag: Create Android Accessories Without Android Code," (slides not available).
They had a mini "Maker Faire" with a load of small companies showing their hardware projects.
Massimo Banzi's slides: .
Massimo Banzi's video: .
Simon Monk's slides of "Building Android Accessories using the Open Accessory Development Kit and Arduino": .
Oh, and the food was exceptionally good, as was the open bar on Monday night. Strangely, some of the food and booze was sponsored by Microsoft, who you would think would not be an enthusiastic supporter of Android.