Just to close the loop on this super popular thread, the new cat that I replaced two weeks ago was rattling like crazy today so obviously it is defective. I think the cat on the passenger side is rattling as well so I will likely replace both of them - one I will have to pay for and the other one should be free since it died after two weeks. The original cause of all the problems - likely 60,000-plus miles with a Vortech blowing up a cats ***.
Just to share what I've learned about the annoying P0420 code:
1 - The catalytic converter may be clogged all to hell or has gone bad (like Evin said). Replace the cat(s).
2 - Your O2 sensors may be fried and your engine is overcompensating what it thinks is a lean fuel mixture. Replace the O2 sensor (much cheaper than replacing the cats so hope that this is the problem).
3 - A supercharger will hose your cats after 60,000 miles without a doubt. Mangaflow, Flowmaster, etc. They will all be dead eventually.
I've probably read every Website post possible on this problem so here it is in a nutshell. Hope it helps somebody eventually.