Arduino Mega ADK demo kit on Xoom and Nexus S
I went back and added some log outputs to the ADK code and figured out that the demokit software was not making it through the setup method. Turns out it was hanging on this line: 

init_joystick( 5 );


I commented this line out and all works well. I don't know if it matters but while I was doing all of this I had no components plugged in to my ADK board.