Gizmo5 is finally the one to break the Skype barrier. They have a new service now called OpenSky (har) that is essentially a proxy between Gizmo's SIP network and the Skype network. I imagine they're doing something like the non-free Skype Asterisk channels that have been out for a while. This is cool but it would have been even more interesting when Skype was offering free out-calling. Now, I'm not sure how big a deal it is.
Ultimately, the SIP world needs to come together, perhaps with XMPP as the discovery mechanism, in order to support universal and open calling. I've been fiddling with IM on domains for Google and it works great! More of that is needed!
And yes, I'm aware of ENUM but that doesn't seem to be making progress in North America. Someone show me that I'm wrong about that.