Once again, let me remind all of you, US iPhone with serial 7R745 and all the UK iPhone are having the new bootloader 4.6. Old iPhone that have boot loader 3.2 can be unlocked while 4.6 cannot. But, this may soon not to be true at all. George Hotz, the one who hardware unlock iPhone, found 2 new exploit in the iPhone.I am not a programming guy, so cannot really understand what is it all about, you may click the following link to know more.

[via Finding JTAG on the iPhone]