As now your tablet/phone is a complete USB host device, so you can theoretically connect to any USB devices avaliable on the market. But you have to write your own USB driver using USBManager (Java) to talk to the devices.
There is a one example provided by Google called ADB Test, so now you can connect your tablet/phone to another android devices via USB and start debugging. (i.e. adb logcat)
Source code: http://developer.android.com/resources/samples/USB/AdbTest/index.html
I got a Asus Transformer TF101, and a special USB kit which turns the external connector to a USB host connector, so now I can connect my phone to it with the USB cable! Let's see the demo video here:
If you want try it on your tablet/phone, you need to prepare a special usb host adapter if necessary and then download the following from the market:
it is a direct copy of the Google Adb Test, I put on market just for people to play around.
I have started a little project of interfacing a USB2Serial adapter (based on SiLabs CP210x chipset), so far I can send / receive the data to/from the MCU UART. I hope can finish some basic functions (like setting baud rates) and release it to public later. It is a simple and cheap way to connect your Android devices to your MCU, the only drawback is that you will need a usb host and Android 3.1.x host device.