Is there a Terminal command to eject disk?

Hold the mouse button down while booting. That should eject the disc.

There is a Terminal command and it does work under 10.5.1 and 10.5.2. In Terminal type drutil tray eject . You can look at the command man drutil and it lists a lot of related commands. I have sometimes had the mouse method fail to work but so far not the said drutil command. I assume there is only one Disk Recorder involved, otherwiuse you'd have to specify the drive to be ejected.

In the terminal, enter
drutil list

you should see the drive listed with a number
Then type
drutil tray eject
or
drutil tray eject 1
or whatever the number of the drive was under list

In researching this I discovered that you can put a permanent eject button on the top menu bar as follows:
go to System->Library->Core Services->Menu Extras
There you will find a veritable treasure-trove of extras, one of which is the eject.menu.
Double-click on the item to install it on the top menu bar.

Extra bonus:
There are many other items in the Menu Extras folder that you can install on the menu bar the same way.
To remove a menu bar item, hold down the Command key and drag the menu item onto the desktop. You will get a "poof" as it disappears. Only the link is "poofed" not the actual item.