Why can't I set GTalk on my Samsung Nexus S to notify when a friend is online like it does on the computer?
The reason for this is because that feature is not included on the application. It is more related to Gmail which gives you privilege to talk but it doesn't notify you of online activity between your peers.
Well the issue of it is that the GTalk don't let you have notificaion, assume that you're using cell phone, and all of a sudden it shows you a bunch of notifications for every friend who gets online!
Secondly GTalk is no more! Its now Google Hangouts, you can update to that one now! Its super. You get your conversations everywhere, Google+, Gmail, Hangouts!