If I am not mistaken the fellows over at the iPhone Dev Team had advised 3g users not to upgrade to 4.0. ~ My apologies... try these instructions http://blog.iphone-dev.org/
I looked the similar question a few days before. when attempting to search the solution I stumbled upon a way to get the new iphone 4g from www.freeiphone4help.tk!!! the phone is scheduled for delivery in 4 days n it's going to be great!!!