I'm planning to connect an Android 1.5 device (nook classic) to the mbed using the USB host in order to add a portable, very low power user interface to my project. I have found several implementations of ADB on microcontrollers ( http://code.google.com/p/microbridge/ ) but has any work been done on ADB on the mbed? (There are some ADK implementations but I don't think that works on Android 1.5.)

ADK supporting the ADB works on 2.3.4 Android onwards and USB host capability for Android starts from Android version's over 3.1 starting, So in a nutshell according to this you have to atleast have a 2.3.4 ginderbread running, you can check my info on developer.android.com

The ADB is a much better way of doing this than the ADK. I'm sorry to say that I've not seen any examples of this on mbed, however having a look at the arduino code for the subject would probably give you some info; or the IOIO website, who are the people who have taken it a real step forward.