That's been getting a lot of attention over the last few days. Should be good for those in Arduino land.
Rob
Accessory mode needs board firmware to be rewritten. USB host API is more preferrable in order to communicate with already existing boards:
But it is not well documented in comparison with accessory mode ;(
The first questions about usb host mode usage will be: http://arduino.cc/forum/index.php/topic,97645.0.html