One day, in the not so distant future, the Conan-Leno feud will cease to be the gift that keeps on giving. Not today, though! (Or this Sunday, either.) David Letterman made a rare appearance on Regis and Kelly and talked casually and at length about Leno, impersonation included. To be fair, Letterman started off the ribbing of his longtime rival with some kind words. In their young-comedian days, Leno "was without question the guy everybody looked up to, everybody emulated." But then Letterman went on to say, "There's also a side of Jay where he's a bit of a knockwurst," call the whole Tonight Show kerfuffle "wildly entertaining," explain that he told Jay that, in fact, Jay and Conan did not both get screwed, and do a pretty sweet impersonation of Jay getting jealous that Letterman got to cozy up to Oprah during their Super Bowl commercial.