Letterman blackout idea

How about this? Tina Fey walks across camera while Dave Letterman is doing his classic back-to-the-audience-lean-against-proscenium-stage-right schtick — and she's wearing waders. When Letterman faces the audience, he doubletakes and Fey says, "Hiya, Dave! Are ya happy to see me, or is that a bong in your pants?"