When first "announced", the Last Day campaign was presented in a way where we figured that the metal COG tags were only given to select media members and the codes (CSIDs) would only unlock various wallpapers, icons and posters on the Last Day website. Well, things are a bit different now (and still confusing), because now COG tags are being hidden all around the globe and it's your mission to find them, enter the codes online and unlock video content on the Last Day website. COG tag locations are randomly popping up on the website in the form of longitude / latitude coordinates, which you can view right here. There's still a bit of confusion and lack of organization, but you can follow all the latest Last Day COG tag news by reading the epicforums, keeping an eye on Gears 2 community manager SixOkay's Twitter and by browsing the official Last Day website. Find those tags!
Source - Official Last Day website
Source - Last Day discussion
Source - COG tag locations (updated frequently)