It was just a few days ago when Taylor Swift announced a massive endorsement deal with Keds, and now she took to a video message to announce to her fans that this time she is signing on with Diet Coke. Taylor's song, "I Knew You Were Trouble" will be featured in TV commercials and online advertisements.  Diet Coke will also be used in social media campaigns with Taylor, as well as tied in with her upcoming Red tour, which begins in March.  There was no information on just how much cash she will be receiving for this deal.