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Seems to be in core (will be in 2.1)
It's working OK in here (vf.org) but the tagged URL is with discussions, not another name, maybe it's a fix in the upcoming 2.1, or that's the changes done to display megaphone instead of discussions that introduced a bug.
Something like the flagging plugin?
They have just developed a compatibility layer for the themes. Check the commits.
I guess vf.org is running off the "live" branch, it was created a while back. Prior to that it was running close to master (I guess it's still close to master).