The CD-Key is not retained on the computer. When you enter it, it is one-way-hashed into your "Product ID". If you call microsoft and provide your product ID they may be able to resupply you with the CD-Key.
Everyone so far that I've seen, though, have suggested that you don't actually need a CD-KEY to install Office on a mac. You just drag the program into the applications folder.
See these forum threads:
http://griffmiester.com/2008/04/17/lost-the-product-key-for-microsoft-office-2008-for-mac/http://www.experts-exchange.com/Apple/Operating_Systems/OS_X/Q_22809449.htmlhttp://forums.macrumors.com/showthread.php?t=362079
