There seem to be an increasing number of internet based VoIP providers (Skype being the best known, Google and MS recently) but I wonder how or if they will interconnect? Surely if they want to really challenge the power of existing telco's they will need to make it easy for users on their different networks to call each other.

The problem seems to be that they use a range of protocols, some open and some proprietary. Is it in their interest to interconnect with each other or are they still buidling their own customer bases?