As has been mentioned, Google Talk's protocol, Jabber, has been supported since Tiger's release. What I am guessing is that with Leopard, it adds "Google Talk" as a pre-configured setting. (i.e. you can pick 'AIM/.Mac', 'Bonjour', 'Jabber', or 'Google Talk', where picking Google Talk is really just a pre-configured Jabber setting.) What I would like, now that Apple is getting closer with Yahoo is to have Yahoo IM functionality. That would give iChat access to all the major IM networks (AIM/.Mac/ICQ, Jabber/GTalk, and Yahoo/MSN.)
And the Wikimedia foundation's dictionary site is "Wiktionary". I doubt the OS X dictionary will look up words in Oxford or Wikipedia (which is an Encyclopedia,) it will probably give you the choice of OED or Wiktionary.