To add to that.... you may hear others saying they want to or have unlocked their Storm's for at&t, but this is because the Storm has GSM frequencies that at&t use in the States. Those are the EDGE 850/1900mhz. This means the 3g frequencies will not work as it's either CDMA 3g or 2100mhz for GSM 3g. 850/1900mhz is what AT&T uses for 3g as well.
Here are a couple links to get some info on the networks from: