This is some sort of an official awnser from WD… Cut and paste from an email I received after asking the question to their tech support:
Thank you for contacting Western Digital Customer Service and Support.
Our internal testing shows that the MyBook World's will transfer at 24-40Mbps (3-5 MBps) on a local network. The drive does not move data quicker because that is the maximum throughput that the enclosure's CPU can handle.
We do not advertise sustained transfer rates on any of our internal or external drives. Instead, we advertise the interface speed that the drive supports. The 3-5 MBps is a sustained transfer speed whereas 10/100/1000 is an interface specification. Similarly, we advertise our internal drives with SATA-150 or SATA-300 interface compatibility but the sustained transfer rate is not 150 or 300MBps. Nonetheless, the MyBook World will show a 1000Mbps interface connection when connected to a router Gigabit-capable router.
Western Digital Service and Support