It is already unlocked. If you insert another SIM card from another carrier, most likely only voice will work.
To get data to work, you need to do some APN setting changes. This requires root and then a quick edit of an APN config file. Then you replace the file in your phone with the one you already did.
XDA developers has the full tutorial on this. They even edited the file with well over 20 carriers so you just download it and then replace the original file in your phone using Root Explorer.