USB storage devices, such as USB sticks and harddrives, are common to attach external storage to any kind of computer. How can the Raspberry Pi emulate an USB storage? The easiest application was to directly access the SD card via USB, but one could also provide acccess to files via Ethernet. I thought about the following layout for an "USB cloud stick":
Computer <---USB---> |Raspberry Pi| <---Ethernet---> Cloud, NAS etc.
The computer should only see a normal USB stick to read and write files from. The Raspberry Pi would act as programmable bridge that map directory listings and file accesss to request to a cloud storage. You could access cloud storage hosters with any computer (including black box media players) via USB without having to install any additional software on the computer. The Raspberry Pi USB bridge should also be able to encrypt/decrypt files on the fly, so one could store files encrypted in the cloud and access them on any device like a normal USB drive.
Edit: Existing products with similar but limited functionality include Wireless Media Stick and USB-over-Network. Access to files in the cloud may be possible by mounting virtual storage with cloud drive or similar software and encryption with TrueCrypt or EncFS - passwords would be stored on the SD card of Raspberry Pi only but one could access the storage with any computer without having to type in a passphrase on a untrusted machine.