If you have already heard about the Google Instant Messanger client Google Talk [1], the domain is already available: http://talk.google.com/ [2]. This sub domain will redirect to http://www.google.com/talk/ [3] like Gmail and other Google Services are redirecting. So, we have to wait... maybe until tomorrow [1]? See more details on Google News [4].
Update: the web site is online since 24th August.