Joystiq has documents and loads of photos that detail mass storage device support for the Xbox 360. Flash drive users will be able to convert a drive 1GB to 16GB into an Xbox 360 compatible device. After creating a 512MB system partition you can use the drive for a game install, profile transfer, and Xbox Live Arcade storage.
The USB storage feature has some limitations. It will not support large external hard drives since there is a 16MB cap and you can only plug in two drives. A system update in 2010, perhaps Spring 2010, will add the USB storage feature and put an end to the overpriced 512 memory unit.