I had this problem when trying to update my iPhone to 3.0. I know a lot of you guys might have the same problem. This is what I did to fix it.

1 – Update your itunes to the newest version 8.2
2 – Update your iPhone to OS 3.0 via iTunes ( just click on update)
3 – Download the 3.0 firmware to somewhere on your computer
Iphone 2G
Iphone 3G
4 – Go to iTunes and restore the 3.0 OS (click on shift+restore)
5 – Once its done, use Redsn0w.

Hope this can help you all…