The USB on the chumby is a "host", so that you can hook up peripherals, but that means you can't hook it up directly to another host such as a desktop computer.
We considered setting up the chumby to use USB OTG ("on the go") which allows the device to act as either a host or peripheral, but OTG mandates the use of the "mini" USB connector and special USB cables. We felt it was more important for the chumby to act as a host and use common cabling and connectors.
Since there's no substantial available storage on the device iteslef, it makes little sense to mount the device as mass storage on a PC, as you would an MP3 player, for instance. Instead, we went the other way - you can mount a PC's shares (using NFS) on the chumby device, and you can use scp or other tools to transfer individual files and directories.