I already have the superdrive in the main bay so I'm guessing I would only need a CD drive in the second one. Anyone have any experience/recommendation with one they liked?
What do you plan on using it for? Even dual layer DVD burners are going for around $30 from a quick glance at Newegg if you have to fill up that second bay...
Even if direct disc to disc copying is all you mostly plan to do with it I would still say go for another burner. They're cheap enough nowadays to where it doesn't make much of a difference between types of drives unless you're talking about HD-DVD or Blu-Ray. Besides, you never know when one of them could fail or you need to burn a CD/DVD image to both drives at the same time. Its not necessary to even have another drive but if you need to get one you might as well go for another Superdrive/DVD/CD burner.
I just bought a pioneer DVR-111D - they are only €30 (better than the €99 BTO option) and it took less than 5 mins to install. System Profiler sees it as apple supplied & supported.