In what could be her final days at CBS since the company recently pulled its daytime talk show offer, Katie Couric last night appeared alongside her CBS colleague David Letterman in an elaborate skit by Stephen Colbert, which involved The Colbert Report star walking onto the stage of The Late Show in the middle of Letterman’s monologue. The skit, which involves a shared bathroom key, starts in earnest after the 1-minute mark and includes a factual inaccuracy. “This network and CBS are owned by the same company, Viacom,” Colbert said of Comedy Central. While they are under the purview of Sumner Redstone, Viacom, which owns Comedy Central, and CBS Corp., which owns CBS, formally split in 2006.