Circuits@Home has been journaling their development of a USB host mode shield for Arduino, which could bring the platform a veritable explosion of mass storage, keyboard input and countless other peripheral options (and a corresponding raft of new hacks). Currently at the prototype stage, a ready-made shield and library are forthcoming, so keep checking back. Meanwhile, for embedded developers wanting to learn the ropes of USB, the in-progress articles will likely provide some valuable insights.

[thanks Kersny]