Danielle Panabaker and Carlos Valdes, (who play the characters that will eventually become Killer Frost and Vibe in the CW's Flash pilot) will appear on Arrow season 2 episode 19, "The Man Under the Hood." The original plan was to have Grant Gustin return as Barry Allen (and possibly the Flash) around episode 19 or 20 this season in order for it to function as a backdoor pilot for the Flash show. But once it turned out that The Flash would be a standalone pilot, those plans had to be scuttled because, according to Arrow showrunner Marc Guggenheim, Barry has "got to stay in a coma until October."
Nevertheless, Panabaker and Valdes (in their roles as Caitlin Snow and Cisco Ramon) are STAR Labs employees, who come to Starling City for reasons that haven't yet been disclosed. "It allows us to further flush out The Flash universe in Arrow," Guggenheim said, "We get to see their dynamic." So, even though this has more to do with The Flash than Arrow (technically) this is still another DC Comics supervillain and superhero who are making it to the small screen courtesy of this show, adding to a season that's already seen more than its share!