if it is locked through an Apple ID, the only way is through the Apple Store Genius Bar, but you will need proof of purchase in hand for them to even acknowledge they can do this (done this twice with corporate phones people did not unlock when they left the company. This is a policy they will not budge on.)
If you are talking about just a passcode on the device, you should be able to unlock it through iTunes, but the device will be wiped.