Happened to us too, they changed how it worked so it doesn't show for all websites[1], just some, but we're still not sure if they took it off or if there's a bug in our code.

Worse still, it doesn't really seem to have affected our incoming traffic, so the whole thing was a complete waste of time and trying to fix it is now very low on the to-do list.

[1] Matt Cutts mentions it somewhere in here http://www.youtube.com/watch?v=K7JhnWHbwnE sorry don't know the exact time!