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"
We will keep you updated.