I have a similar problem, I can't find a away to activate my iPhone 3GS 4.0.2!!
I was running 3.1.3...I did a restore to 4.0.2...I have all the tools I have to make it run again...nothin just work to activate it!!
tried Redsn0w and Sn0wbreeze (latest) they both fail to identify any IPSW below 4.1!