I bought a new 8GB iPhone to replace my 4GB one and it worked great. Just go to AT&T, ask for an iPhone, pay for it and take it home and run the code. Works great for me too (although the pseudo-GPS doesn't work with the new unlock and you need to use the "Locate Me" app instead).
All credit to George Hotz and Zibri. Impressively fast work.
George talks a bit about how he did it here:
http://iphonejtag.blogspot.com/