See Why does the N770 have Google Talk instead of Skype? on Robin Jewsbury’s Forum Nokia Blog. I do share the opinion that SIP is going to be a mainstream protocol for Internet Voice [and Video?] applications, though there are other opinions (including SIMPLE, IAX etc.). It does not however seems to appear much too fast since as we can see XMPP is very slowly becoming popular too.

There is no doubt SIP will be the underlying technology of all P2P systems in the future and no doubt in my mind that Skype will have to move to SIP at some stage. SIP is far more efficient than Skype.