To me, the interesting points are:
1) It’s not SIP. It’s based on Jabber, and the Jabber XMPP protocol. What does this mean for SIP?
2) It’s completely open. Any Jabber client can connect into it, which means that the business model that AOL and MSN have used (fund IM with advertising dollars) is not the business model which Google is using initially. So, what is the business model?
A reasonable supposition is that Google is trying to dominate the directory space as effectively as it has dominated search. By linking GMail and Google Talk, they’ve given you more reason than ever to use GMail, and to invite your friends to use GMail.
The folks at Skype are likely having kittens over this. The tight integration with GMail will mean an instant, and large community. The people who really need to worry about this are the ILECs. It is they who control the dominant directory assets today.