Open a client supporting Jabber (AdiumX, Gaim, etc.)
Your server name is talk.google.com
Your username is the same as your Google ID. Example: firstname.lastname@example.org
Your password is the same as you use to login to Gmail or other Google login locations.
Update: And now the talk.google.com website is up offering a Windows Jabber client for download, as well as Jabber-compatible instructions for Mac & Linux users.