Apple has this weird habit of generating multiple part numbers for what is essentially the same device, probably to track sales and service through different channels (retail, carrier, direct, etc.).
If you had purchased an AT&T iPhone 4 (Black) from Apple at launch in the U.S., you would have received either MC318LL (16GB), or MC319LL (32GB). However, if you had purchased the same phone from AT&T directly, you would have received either MC608LL (16GB), or MC610LL (32GB).
They are the exact same phone. The different part numbers just indicate whether it was sold by Apple or by AT&T. And Apple has been doing this since at least the iPhone 3G.
If you have your AT&T iPhone replaced by Apple under warranty, you will receive an MC318LL or MC319LL as your replacement, regardless of the original part number, because Apple only carries the "direct sale" part numbers on-hand; all of the AT&T-specific parts are allocated to AT&T's sales channels. But -- again -- it doesn't matter, since it's the same phone.
The unlock status of your phone also has virtually nothing to do with the phone's part number. Phones are marked locked or unlocked by Apple based on the IMEI of the phone, in a master database they maintain that iTunes has access to. When you activate an iPhone either through iTunes or through the new iOS5 on-device activation, that IMEI database is checked, and the phone baseband is locked to whatever carrier that database says to lock it to, OR if the database says that IMEI is unlocked, then the baseband is unlocked. That's actually a very simplified explanation of the process as it's not just a matter of "locked or unlocked"...the lock status is protected by Apple cryptographically, so you can't just "fake out" iTunes and trick it into unlocking your phone for you. Also, unactivated phones are not (contrary to some rumors) unlocked until they are locked by the activation process...the baseband is a blank slate until activation, which means that it will accept NO SIM until it is told what it is allowed to accept. So you can't buy a subsidized phone, try to avoid the activation or "hacktivate" it instead, and expect to come out on the other side with a fully-unlocked phone.
So when you get a warranty replacement from Apple, Apple looks up the lock status of your old phone and applies that same status to your new replacement phone; the model number of the replacement has no bearing on things at this point. Apple does it this way because even phones that were originally purchased as locked can be unlocked at a later date: the carrier that sold the phone merely has to authorize the unlock, and Apple will change their master IMEI database to reflect that request. And if one of those locked-then-officially-unlocked phones needs warranty service, the unlocked status should follow it (and it does).
When a "factory-unlocked" iPhone 4 model was finally introduced in the U.S. this past June (sold, as far as I know, only directly by Apple through their online store or retail stores), the black models were given the part numbers MC603LL (16GB) and MC605LL (32GB). I don't know what Apple would give you in its place if you had purchased one of these unlocked SKUs and then you found a few months' time later that there is a warrantable issue -- would they give you another phone of the same SKU, or one of the sold-by-Apple-but-locked-to-AT&T SKUs? -- but it really doesn't matter because there is absolutely no physical or software difference between ANY of these phones. Everything is controlled by the activation process and that IMEI database.
-- Nathan
