"Marvel's The Avengers" -- succinct title! -- arrives in theaters in just over three weeks, which means it's time for the marketing to kick into overdrive. Earlier this week, star Cobie Smulders (Robin Sparkles, if you're nasty) revealed a new clip on "Late Night With David Letterman," and now comes this brand-new video from MTV.com showcasing Robert Downey Jr.'s Tony Stark and Tom Hiddleston's Loki. In the 64-second segment, Stark and Loki have one of those mutually-assured destruction conversations...