According to MuscleNerd you can't use TinyUmbrella-like backup of signed BB blobs because they're unique *per restore* (!!)
the blobs are only good until the next time the baseband reboots. Then the nonce changes and blobs worthless :(
MuscleNerd hopes that there's a flaw somewhere in the iPhone4 at+xnonce stuff which will allow the Dev-Team to circumvent Apple's security measures.
Also the iPhone 4 Bootloader and Bootrom has yet to be explored for exploits that might allow baseband downgrades.
Currently this feature is still a welcomed one for developers who were previously forced into a baseband upgrade when testing out beta firmware. Now they can restore back down and get back to the older baseband.