For the longest time, the one thing I’ve liked about Windows Live Messenger over Google Talk is the option to “Appear Offline.” There are times when I’ve wanted to set myself to “offline” in Google Talk without actually being offline for those instances when I’m really trying to focus and get something done and I don’t want to be interrupted. Up until recently, the only thing I could do was set my status to “Busy” but even then, people still talked to me to see if I really was busy. Google has partially solved my dilemma by adding an “invisible” option.
This new “invisible” option is found only in Gmail Chat (not Google Talk) in the drop-down menu with the other status options. Once you click “invisible,” you will appear offline to your contacts but you’ll still be able to see who is online. You’ll also be able to talk to someone even with your status set to invisible, which is nice.
For those of you who only use Google Talk and would like a similar feature available to you, there’s a Freeware application called gAlwaysIdle which will allow you to always appear Idle. It’s not exactly the same as being invisible because you’ll still appear online, but it shows that you’re away from your computer. The only downside is that at this point, it’s available only for Windows. A screenshot below shows the menu options you’ll see once you install gAlwaysIdle.
Source: Lifehacker and ZDNet

It’s about time if you ask me. Every time I would go invisible with Pidgin I would be offline with GoogleTalk, now I don’t have to.
In Gmail I have had the option to log out of chat for at least 6 months. True, it’s not invisible which would give you the option to chat if you want but it worked. That has been my quick and dirty work around, but I am glad to know we now have new options.
I’ve used the “turn off chat” feature before as well, but then you can’t see who’s online and you don’t have the option of starting a conversation. I’m just glad they finally added the invisible option.
you can also find some extra features on the labs version of other Google products which allows you to make yourself invisible but it does not always override the gtalk client software if you have that installed
[mwolk.com]