Here are some facts about IMEI blacklisting. Typically US carriers do not do it. EU carries maintain and, as far as I know, share a blacklist for IMEI's to prevent people from stealing a phone and using it on another network. US carriers do not share such a list. As far as I know, they usually don't even bother to blacklist a phone at all.
When a phone is stolen and reported, they de-activate the sim card. They do not do anything with the phone, however. So no charges will incur on the account, but the phone is free-game, unfortunately.
ExitRightHere - With the hassle involved in actually getting it done, I would advise you not to recommend that to victims. I would suggest you recommend they call their service provider and the service provider will de-activate the sim card, preventing the theif from racking up their bill.
Now it's been a while, but I think Verizon maintains a blacklist of ESN's (CDMA IMEI's). I haven't looked into Verizon in about 4 years though, so I don't recall if that is indeed correct.