Trying to unlock my iPhone 4 really need

a SIM from another carrier to complete the unlock. I have my email from ATT. I've erased the iPhone etc… leaving my ATT SIM in. But no sucess of unlock in iTunes.

I called ATT, they switched me to APple and said I need another SIM to complete it.

I'm going to the UK, in December and she said if I followed the procedure today, when I get to the UK, insert a SIM and the iPhone will work. BUT when I come back to the US, I will have to do the same procedure using iTunes again and insert my ATT SIM to get it to work again in the US. Seems strange.

SO the wuestion is do I need another SIM to get this iPhone unlocked now in the US and use as I do now?

Thanks all.