[fishingmedic]

Sim unlocker will allow you to pop in GSM sim card and use it in the US, but you can only flash CDMA ROm's and radios or you will have a nice new paperweight, as Kliptik noted. Read the thread the doc noted about the unlocker and if you're still not sure, post you're questions. We'd rather you ask the questions and be completely sure about what you're doing, than post that you just bricked your phone.