Google Talk is up and running
… but not, really, for Mac users. But no surprises there, I guess! Still, it does work in Adium, and apparently iChat, except for voice. (Which is something I wouldn’t be using anyway.)
You can download it here (900K) and give it a go. (You’ll need a GMail account. If you don’t have one, ask me. I have a zillion invites.)
Update: GMail has apparently been made free and available to all to coincide with the release of Google Talk, which requires a GMail account and draws the contact list from your GMail contacts.