Orkut has now added fully integrated version of Google Talk that works seamlessly from within orkut, with no install or separate window required, chat just like Gmail.
Its got its own setting page to set your preferences. Lets you decide who you would like to chat with.
To sum up, the new Google Talk in orkut lets you:
* chat with your orkut friends in real time from directly within orkut
* control which of your orkut friends can chat with you
* chat with your Google Talk and Gmail chat contacts
* visit different orkut pages while keeping your chatboxes open