If you are talking about configuring a combo drive on 20" iMac, or a SuperDrive on 1.6 Ghz 17" iMac, you are temporarily out of luck.
But I am sure that someone out there might be willing to trade the drive with you (plus or minus some differences). Or take it out and sell it if you don't want the drive (probably not a big market for combo drive). Just post a trade message in the buy/sell forum.