Google introduced a Google Talk gadget into Gmail quite a while ago, but some people didn't like that signing into Gmail also meant signing into Google Talk. For them, Google added a feature that lets you "turn off chat" by clicking a link at the bottom of the page.
Though this was an easy way to solve the problem, this left a subset of users who like being logged in (but invisible) out in the cold. Most popular chat clients already support that kind of functionality, but for some reason Google Talk always insisted that you were either online or offline -- no way to be invisible.
Even though the Google Talk desktop client still doesn't support invisible mode, the client in Gmail now does. Now when you log in, you can see a list of people available to chat, and you can remain unseen.
This could be a devil in disguise though -- as more people use this setting, there will be less people in your contact list available to chat. Since most Google Talk users use it through Gmail (take a look at your contact list and count how many people don't have a phone beside their name), this could quickly become a problem.
Even though there is some level of risk for Google, it's probably a good thing they added this feature -- it satisfies a user need that is standard everywhere else. What do you think? Will this new feature affect their user-base in a negative or positive way?