Interesting announcement at Google IO last month about support for usb devices connecting to Android phones and tablets (so you could plug a game device in to control it), and an open way of developing custom devices to plug in called the Android Open Accessory Development Kit based on the open source Arduino platform.
Rather than making people sign non-disclosure agreements to find out the hardware details (like another well known manufacture of tablet and phone devices), and making them pay large licensing fees, and submit devices for approval they are basing the initial development kit on a popular, well used open source project that anyone is free to make. A number of low volume manufacturers are making boards to use already.
There were already a ways to connect devices to Android devices, but none were officially sanctioned, so could be blocked or changed at any time. Having an open route that is officially supported is great. On the day of the announcement they made all the details available, including the design files you need to make your own (one of the requirements that makes hardware open source)
While there are some quirks in the way that they are doing this is an exciting development and could lead to a lot of interesting devices and prototypes.
I have got as far as getting the software example that talks to the board onto an Android tablet – as my tablet didn’t have the latest version of Android on it I needed to update it. As it is open source there were versions online i could try (legally).