Apparently the Touchpad has functional USB Host support out of the box, i.e. mouse/keyboard/HDD and other USB devices. You do need to provide power from a self-powered USB hub.
All credit goes to this place, I'm just reposting since I didn't see it anywhere here.
I wonder how this works for USB drives, cameras, and so on. Could you just plop movies on an external drive and play them on the TP? Can I dump pictures taken with my DSLR onto the TP to review them on a larger screen? I need to try this out .