It's a common problem with optical drives in general, and the last 20" model Apple made was in Early 2009, that puts it at a minimum of about a year old, assuming you bought it close to the time to Late 2009 models came out, which is right about on schedule for when optical drives start failing.
The good news is, this is a pretty easy thing to fix, and if you're daring enough, could probably even do on your own. The bad news is that the iMacs use laptop style slot loading drives which aren't exactly cheap. If you buy just the drive from Apple, expect to pay around $200 for it. So, unless you really need that disc out of the drive, probably best to just sacrifice it to the cause and get an external DVD burner. I've seen some for as little as $50, so you could go through 4 of these for about what it would cost to get a single internal replacement.
Of course if you bought the AppleCare extended warranty, and it's still valid, take your system in to get an AASP or Apple store to fix it. They should be able to disassemble the drive and extract your disc as well.
Had been making some strange noises when not in use. Put in blank cd but the drive was not recognized nor can I remove the cd. OS 10.6.4 snow leopard. Anyone had similar experiences with this problem? Drive might have died. Is this a common problem with the superdrive? Thanks.