I all i found this out last night while trying to get gpsphone to work on my 3g.
First off what you need is:
Iphone you want to downgrade.
ireb found at ih8snow.com
custom FW - google is your budday!
,so far ive gotten my 3.1.3 down to 3.0. : )
Alright so first of you need to put your phone into DFU mode. After that fire up mister Ireb. click on the button under the idevice you have, wait a few seconds. Your iphone will bring up this white white screen. click on the open itunes button. Shift click restore and select your FW, Now your FW can be any version. Here is the tricky part once it runs and restores it it will tell you to restore your phone, Unplug it from your computer turn it off then back on. It will boot up and run like a charm
All this i found out myself if its already some where and you say im stealing it suck it. haha
DOES NOT DOWNGRADE YOUR BB!
REPEAT DOES NOT DOWNGRADE YOUR BB!