This is a screenshot from Adium, an AIM & Jabber client for OSX. As you can see, it logs the time that any of your contacts sign in or out. This particular contact is using the GTalk network. The problem is that GTalk is often (as in the case above) used within the context of the Gmail client, not a third party app like Adium. The log effectively tells me every time that they check their email.

The question, is whether or not this constitutes a privacy concern. No nefarious uses jump out at me, but there must be cleverer people than me who could exploit this information. One potentially useful application (for the paranoid) is to add your own GTalk account as a contact, which would show you if anyone else has been logging into your email.

For those people who do view this as a problem, the solution is to turn off chat in the Gmail web client. The option appears in the page footer (as of this post).