If you have AT&T and T-Mobile, it was easy for a thief to steal your phone and then pop the SIM out to resell it to someone else. Sprint and Verizon are more difficult, unless they are new LTE devices with a SIM, because they are provisioned through the carrier. Thankfully, that all changes now as these four major carriers turned on the joint database to track the IMEI numbers of cell phones, which is specific to the hardware and not the SIM card.
While this won't likely lead to the recovery of your stolen phone, the hope is that by having the ability to prevent phones from being activated with a stolen IMEI the stolen cell phone market will dry up and die. There may still be a market for stolen phones by selling overseas, but in most cases we see the modern smartphone launch first overseas before the US so that may not be much of a market either.
I have spent thousands on smartphones and welcome this new initiative from the CTIA and am pleased that at least the four major US wirless carriers are on board.