Mass storage devices like external harddiscs, flash sticks and card readers
should work out of the box, since support for them is enabled in a stock
MyBook World.
However, for support of CD-ROM or DVD drives you need to compile the appropriate
kernel module(s) first.
Due to a bug in the USB root HUB transaction translator of the EHCI controller
(or somewhere else ?), low speed devices will not work when connected directly
to the MyBook World.
But if you connect them with any USB 2.0 HUB (well, one with a working transaction
translator ;-), the device will work!
I did not get those to work, since the usb-alsa kernel module does not load because of missing symbols related to the PCI bus.