Logged on this morning via Messages and automatically expected Mountain Lion to be the cause but it turns out Google has a systemwide outage for Gtalk
http://techpp.com/2012/07/26/google-talk-goes-down-suffers-outage-across-the-world/
http://techpp.com/2012/07/26/google-talk-goes-down-suffers-outage-across-the-world/
Googles GTalk users across the world are facing downtime at the moment. People using the popular messaging client are having variety of issues, ranging from inability to access their contacts to messages not being sent. The issue is present on Gmail chat as well as GTalk app on android.
Googles App status Dashboard confirms that Google Talk is facing issues and currently down. On the app status page for Gtalk, Google has confirmed that