Heard on the This Week in Tech podcast. There is apparently a bug in Google+ that allows a Google talk user to intercept your IMs if they log into Google talk on the same computer where you had an active Google+ session if you just close the browser tab without logging out.
Full details here. Google thinks it is pretty unlikely that many users will encounter this issue but better safe than sorry.
(Note: Google says that the bug is fixed but since it is still be reported as active today take some basic precautions)