Battery level on Asus eee pad Transformer with dock
The Eee Pad Transformer will use the battery of the dock to both power and charge the tablet so long as it is connected. You can see this when docked and on battery because the tablet shows its battery as "charging". This gets the most use out of the tablet because when you undock it, it is most likely to have more charge than the dock. 

When docked and on the charger, I am fairly sure that both charge at the same time. However, I haven't checked as my TF is docked most of the time.

Incidentally, if the dock's battery goes flat, it gets its power from the tablet (without trying to recharge its own battery).