Google has made some improvements to the Gtalk integration within Orkut to make it "spam free" and slightly intelligent.
Google mentions in a blog post that they have come up with a formula that automatically predicts which of your Orkut friends you probably want to chat with, and only tells those people that you're signed into Google Talk. This automatic filtering of contacts is dependent on your account activity, recent scrap conversations and the category of friends you have added the person to.
For example, a best friend with a recent scrap history would be able to see you online because Google thinks you will obviously want to chat with him. This, however, is not a great idea if you do not want to bump into someone with whom you had a not-so-memorable recent scrap conversation!
Google has, of course, provided the option to customize your contacts and you will have the final say on who will be able to see you online. The enhancements also enable you to chat within an open Orkut window. These tools though have not been fully implemented in all accounts as of now.
For more details on how this works, have a look at the video below...