Some iPhone 3GS 8GB cannot be unlocked with iPad Baseband 6.15.00. Those iPhones are produced with Toshiba baseband chip instead of the original Infineon chip. Therefore it's not compatible with the iPad baseband. If you attempt to flash it with the iPad baseband, you are running the risk of corrupting the baseband and bricking the phone. You will end up with NO wifi/bluetooth/imei after the baseband is corrupted. Restoring the phone in iTunes will result in error code -1.
iPhone 3GS 8GB that is produced before 2011 are safe to flash. However, certain ones produced in 2011 that is after week 28 are not safe. But because phones come from different factories, you mileage may vary.
Some confirmed safe to flash begin with:
Confirmed bricked serials begin with:
The list will be continuously updated. You may post your serial and your unlock experience in the comments and it'll be updated here.
Some folks are asking about if there is a possible solution. At the moment, if you are unlucky enough that bricked the phone. There's no way to recover. However, there is a software based baseband erasing/flashing method that is actively under development. This method may be able to re-flash the corrupted baseband. Once the developer releases the software, we'll post it too.
You may also try your luck at apple store genius bar. In general, attempting to flash the baseband will void the apple warranty. But YMMV.
For iPhone 3GS 16GB or 32GB, Apple stopped producing them in 2011. However, apple may still refurbish them so some 3GS 16GB/32GB may still have a serial number greater than week 28. We have reports that some iPhone 3GS 16GB/32GB with serial number greater than week 28 are bricked.
One anonymous reader tipped that for most accurate result, it is advised to open up the phone and check the id on the baseband chip, if it is 36MY1EE it will work, if it's anything else, it will brick. Thanks!
It's confirmed that the baseband downgrade will NOT work with the toshiba baseband. So if your phone is bricked due to flashing a 6.15 baseband, you cannot recover it via baseband downgrade.