From the CADTutor FAQ 74 (This FAQ is not yet live):
If the Command Line at the bottom of the AutoCAD drawing edit window has disappeared, it can be restored by one of the following methods:
Conversely, if the Command Line is not required, the above commands can also be used to toggle it off.
- Press Ctrl+9
- Type COMMANDLINE (the text will not be displayed on screen)
- Go Tools > Command Line
This command was first introduced in AutoCAD 2006.