Very few can be unlocked these days. From what I understand, if the card has a dual-bios switch, then it has a chance to be unlocked safely. I believe you can try to unlock one without a dual-bios switch, but you may brick it.
Also, it must be a 2GB version for it to be unlockable.
Example of a dual-bios version: http://www.newegg.com/Product/Product.aspx?Item=N82E16814102945