Developer of popular TinyUmbrella application has confirmed an interesting development on Apple's strategy to block carrier-unlockers, starting from recently released iOS 4.2.1 Apple seem to be enforcing a minimum baseband (bootrom) version requirement for users trying to upgrade their firmware. Users have resorted on tools like TinyUmbrella and Sn0wbreeze to update their device firmware to latest version keeping an older unlockable baseband version letting them enjoy the newest Apple iOS version while retaining a carrier-unlocked phone, this has changed for now until someone hacks their way-in finding a new way to get around this. The worst part is that if you try to use an older baseband updating to stock iOS 4.2.1 you will be stuck in recovery mode with no option but to restore back to iOS 4.1 or iOS 4.2.1 which means no carrier-unlock.

TinyUmbrella developer Semaphore's warning to users :

UPDATE: Do not update to 4.2.1 for the baseband protection. TinyUmbrella is no longer able to kick you out of recovery for that version. TinyUmbrella still works for prior versions and will be able to restore and kick you out of recovery for those versions but 4.2.1 has a baseband version check that will stop your device from booting if your baseband was not updated.