Restrictions regarding Google Talk
Recently, Google started blocking subscription requests from other servers to their XMPP-based Google Talk network. Hence, users on non-Google servers can add Google Talk users to their rosters, but they won't see whether the Google Talk users are online or not. However, nothing changes for Google Talk users; they are still able to see the online state of all users on non-Google servers on their rosters.
Users on non-Google servers that have added Google Talk users in the past are only partially affected: They continue to see the online state of the Google Talk users that have already been added. However, the online state of newly added Google Talk users won't be visible as well.
Update: The restrictions have been removed, communication with Google Talk users works as expected again.