SIP stands for "Session Initiation Protocol" and it is a key protocol that allows for open internet telephony. Google Talk makes use of this protocol, and consequently it allows you to connect to other SIP providers like VOIPbuster.
Skype is the big, closed and extremely popular internet telephony service that everybody loves to hate. What I would love is when Google made a service available whereby people could connect never mind if they use Skype, Google Talk, VOIPbuster or whatever other service... there are still people who use only POTS.
The benefit would be a much improved telephony service and it would certainly stimulate improvements to the quality of service.. I would love it when the lag on Skype chat would go the way of the dodo.