It's up on Kotaku, so I assume it will move its way to io9 in the near-future.
This is what it looks like:
Looking around Kotaku it's only in place on Kotaku-main, not the sub-blogs of Kotaku.
Looking around, Deadspin, Gawker, Gizmodo, Jalopnik, Jezebel, and Lifehacker don't appear to have flagging, so Kotaku seems to be the testing grounds for this feature.
Fellow Kinja users, what says you? My quick humble opinion is: Flagging is good because it leads to eventual comment moderation, but bad in that it gives more work for the mods. If the point of Kinja is to elevate the discourse, and there's elements of the Internet that are always uninterested in elevating the discourse (spammers and hateful individuals of various sorts), then flagging is a step in the right direction but maybe not a powerful enough step.