Today is Rihanna's 24th birthday, an event she already started celebrating last week with a big party, a cake in the shape of a giant spliff (with her riding on it), and, uh, Chris Brown in attendance, fueling the rumors that Brown will really be appearing in the new version and video of Rihanna's "Birthday Cake." Well, here's more evidence: Brown and Rihanna are publicly tweeting birthday wishes at each other. This morning Brown tweeted a happy birthday to Rihanna (real name Robyn Fenty) and she replied back: "Thanks! RT " This isn't quite as alarming as Rihanna's tweet last week that " : HAPPY BIRTHDAY ROBYN!They can say whatever, Ima do whatever... No pain is forever," but it is looking increasingly likely we're all going to have to deal with a Brown-Rihanna collaboration whether we want to or not. Happy birthday.