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I tried to read what you linked to, but there wasn't anything.
The dev team says no. They're the ones who utilized the iPad bb unlock. That forum you linked to is a bunch of app pirates. They've never done anything useful in the jb/unlock community.
I trust the people who actually know what they're doing.
From the devteam blog:
There is no way to come back down from 06.15, and there’s no hiding the baseband version from Apple. You’ll be voiding your warranty in a very obvious way.

http://www.sinfuliphone.com/showthread.php?t=52413

Did a little digging and they claim to be able to do it.:confused:
 
I just love how people have this amazing memory for a very unique situation (the 5.08 bootloader ----VERY RARE, and the iphone 3g combo that produced a method for downgrading) that occurred almost 3 years ago and still drudge this up every time someone talks about downgrading, even though it doesn't apply to their newer iphone model.

People: forget that you can do this already! It hasn't applied to iphone owners for years now. Even when it did, it was for a very small group of iphones. I purchased my 3g within a couple days of launch at an apple store and it already had the 5.09 bootloader.
 
I didn't even know this was an option--however unlikely it was. Okay, so there is an exception to downgrading the 6.15bb, but only if you have a 3G on the 5.08 bootloader.
Since this doesn't apply to the OP, my original statement still stands.

I just love how people have this amazing memory for a very unique situation (the 5.08 bootloader ----VERY RARE, and the iphone 3g combo that produced a method for downgrading) that occurred almost 3 years ago and still drudge this up every time someone talks about downgrading, even though it doesn't apply to their newer iphone model.

People: forget that you can do this already! It hasn't applied to iphone owners for years now. Even when it did, it was for a very small group of iphones. I purchased my 3g within a couple days of launch at an apple store and it already had the 5.09 bootloader.
 
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