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Are you not using an Apple keyboard? Holding F12 for a couple seconds should work. Clicking on the disc and pressing Command-E should work, too. You could also just drag the disc to the trash. Some of those may be obsolete on 10.6 though, I'm not sure.

EDIT: beaten ^^^ :)
 
I don't know about a keyboard shortcut, but here is a solution you can use with your mouse.

Enabling an Eject menu You can enable another means of ejecting disks by opening the folder System/Library/CoreServices/Menu Extras and double-clicking the file "Eject.menu". An eject icon will appear in the menubar that can be used to close and open selected optical drives.
If you no longer want the Eject menubar item, hold down the command key while clicking and dragging the icon out of the menubar space.
 
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