i never got this message, not even 10 mins ago when i restored mine to stock 4.0. where did you get your ipsw file from? was it from apple, or did you find it somewhere on the internet? mine was directly from apple a couple weeks ago when i jailbroke for the first time. maybe you file is bad.
i have an idea, but i need a volunteer. i will email someone MY copy of the 4.0 firmware and walk you through the JB process with that.
my device in JBed, with firecores package installed.