Time Machineis page in System Preferences shows the time for the next scheduled backup. However, if the power cord is disconnected, that time disappears and no backup is scheduled.
As soon as the power cord is re-connected, the next scheduled time reappears.
System Preferences: Time Machine (battery disconnected)
This is no doubt a safety mechanism to make sure the MacBook/Pro doesnit run out of power during a Time Machine backup, especially for travelers using a bus-powered external drive. Attention to that kind of detail is a welcome sight in Leopard.