Cartoon Network/Adult Swim is about as mainstream as any anime not aimed at children will ever get. Also, it's still airing on CN/AS, so it can't be that.
I doubt they'd give the second season announcement to the US, and the fact the panel's title includes the word manga, suggests it won't be to do with the anime. So it's probably either another adaptation, or Kodansha will be printing an A4 sized version of the manga (the "biggest" version ever released)...
If it was an adaptation, I could see AMC trying to do something with it, they need some new hits that aren't spin offs and Kodansha are very insistent on using that io9 quote about it being "Japan's the Walking Dead" (despite the fact it's not really anything like The Walking Dead, neither comic book nor TV series), so maybe AMC are piloting it.