Kutcher taped an appearance for Letterman's talk show Wednesday, and the actor came on stage wearing a helmet and a bulletproof vest printed with the phrase, "Not Dave."
Last week, a contributor to a jihadist website posted a threat of violence against Letterman because the comedian had made fun of Al Qaeda leaders on a previous show. Letterman, back from vacation Monday, played the threat for monologue laughs.
During Wednesday's taping, Letterman asked Kutcher if his bowtie was bullet-proof. The actor replied that "you can't be too careful" sitting next to a guy facing a threat.