The most common and elegant way of achieving this through a CUBE. You need to have a SIP trunk configured between a CUCM and the CUBE and another SIP trunk from the CUBE to Skype. This is required because the CUCM does not support SIP Register.
But in a recent thread one of the customer's reported that he was able to have the CUCM configured with Skype without using CUBE. Please check the last post on this thread: