Google Talk is now available for download (and trademarking -- hello, Google?); as expected, use of the service requires a user account for Gmail (trademark application nos. 78,395,746 and 78,398,233, if you're keeping track). If you do not have an invitation to join Gmail, let me know; I (and most other users, probably) have 50 or so to pass around. You should be using Gmail anyhow, and this is a perfect opportunity to start if you haven't already.
UPDATE: Google-owned Blogger's profile function does not allow the selection of Google Talk as an IM contact choice (although competing services ICQ, AIM, Yahoo!, and MSN are supported); I'm not sure why this surprises me, considering that the words "Google", "blog", and "Blogger" are not recognized by Blogger's spell checker.