Wahaab JB at Chip Hazard tells us how it can be done. Here it how it goes:
Step 1 : Launch Cydia, Navigate to Manage>Source.
Step 2: Add the iparelhos repository by selecting Edit>Add, type http://repo.iparelhos.com and then press Add Source.
Step 3: Download the latest official version of Ultrasn0w from Cydia.
Step 4: Now search for Ultrasn0w Fixer for 6.0 utility in Cydia. Tap the Install button and let Cydia work its magic.That’s it!, by following these simple steps you’ll have an unlocked iOS device ready to be used by any GSM carrier in the World.
via Chip Hazard