MuscleNerd has announced that an upcoming update to RedSn0w will resolve issues for those who were unable to downgrade from the iPad baseband.

"ok, figured out why some 3GS phones refuse the iPad downgrade! Can fix that after receiving another loaner (delivery expected on Thurs)

it's due to variations in the NOR chip. The payload can be changed to handle the differences but I need one in hand to test"

You can get more information about downgrading your iPhone 3GS from the iPad baseband for unlock here. Tutorial instructions can be found here.

We will keep you updated.

via iclarified