Gary Burd, software engineer at Google writes in the Google Talk Blog: “I flipped the switch to connect the Google Talk Service to the public XMPP network this morning. Google Talk users can now chat with users on other XMPP services and vice versa”.
On the official Google Blog this is described as Open Federation. While many Instant Messenger and VoIP services only allow you to talk to people on the particular service you have an account on (MSN Messenger, Yahoo! Messenger, AOL Instant Messenger, etc.), open federation lets you choose your service provider and lets you talk to people on any other federated service.