Are you talking about a Blackberry Storm or Tour? Because they are GSM compatible they do use SIM cards while roaming internationally. You can have the device unlocked for you at no cost, however you need to meet certain requirements. You need to have your line of service active for at least 6 months, have not been interrupted for non-payment or have a reversed payment on your account.
However, if you're thinking about unlocking one of these devices for use here in the US, good luck. The frequency bands aren't always the same here in the states. The devices were made for frequencies they use primarily overseas.