How are you gauging the speed? Can you tell its running at fast ethernet somehow? I have heard these are really slow due to the CPU rather than the network. Having said that, Gbit ethernet should be auto-negotiated requiring you to do nothing, so back to my first question, how do you know its not Gbit ethernet?
I would check to see if your router firmware is current. Also, check to see if there are updates for the WD mybook.
I should clarify, when I said CPU I meant the devices onboard ARM processor. If you read the reviews on this device the main problem with them is slow read/write speeds. Just because it is slow doesn't mean it is not running GBit ethernet, which is everyones point.
My Linksys Router (WRT610N) is able to identify the speed by showing a different color for each connection. Blue for 10/100 and Green for Gigabit. Firmware is up to date on the Linksys and Mybook (World Edition II).