Yup, had the same problem. you need to download the firmware from
here 4.0.1 (no newer than 4.0.1)
Launch itunes on your PC or Mac
follow the instructions
here to put the iphone into DFU mode (Device Firmware Update) and use the instructions in the first link to load the firmware onto the device.
You now have a non-jailbroken iphone running a jailbreakable firmware.
Jailbreak it as normal using jailbreakme.com. Once you have Cydia on it, install ultrasn0w from the list of cydia apps and you have an unlocked, jailbroken iphone which is hacktivated (no need to connect to itunes to change sims)