Google Talk is acting while other VoIP players are talking. One of the two major barriers that prevented mainstream developers from writing VoIP applications is now removed. Google is bridging the isolated VoIP islands by opening up their GTalk network to server federation.
Server-to-Server Federation instructions on the Google Talk Developers page.
Community reaction on the google-talk-open news group.
There are apparently initial connectivity glitches with some XMPP servers, but they seem minor.