Apple has once again raised the stakes in their continuing cat and mouse game with unlockers by implementing a boot level signature check in iOS 4.2.1 that will cause the device to fail to boot if the baseband firmware is not correct. This is clearly a reaction to TinyUmbrella's ability to block baseband upgrades on the iPhone 4.
If you try and implement TinyUmbrella when installing 4.2.1 you will be thrown into recovery mode without the ability to kick yourself out of it unless you install the new baseband or downgrade back to iOS 4.1 (if you've saved your SHSH blobs).
If you plan on unlocking your iPhone it is recommended that you hold off on installing 4.2.1 as there is no current method available to maintain an unlockable baseband. Once the unlocking tools or custom firmware utilities have been updated and released it will be safe to upgrade.
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