I don't think this was always true, nor do I recall an announcement, but Google has added Gmail video chat capabilities to Google Apps accounts, including the top-secret free accounts.
Update: I was hoping all members of a domain would be "trusted" for Chat purposes, but that's not so. You still need to use Gmail's awkward UI to establish a "trust" relationship (invite to chat).
Also, this must be pretty new, because the Google App admin page still (incorrectly) says: "To use Chat, users must download Google Talk (Windows only)". Not so, Google Apps Gmail chat works fine in OS X.