Nerd Vittles Asks “Why Wait For An Asterisk to Skype Gateway?”
Ward Mundy over at Nerd Vittles has a great post describing how to build your own Asterisk-to-Skype gateway using the readily available SIP-To-Sis software. They note that this is the core of the recently launched Gizmo5 OpenSky service. If you have a local instance of Asterisk and like DIY then this is a great overview of getting it built.