death tweet
n. A death threat written as a Twitter update.
Other Forms
Vermont comedian Chris King was arrested Oct. 8 for tweeting: "I am dying inside. And I am plainly stating to you that I am going to kill the president." Such "death tweets" on the social media network Twitter have figured in several high-profile threat arrests.
—Patrik Jonsson, “Threats against Obama: Michael Stephen Bowden is just the latest,” The Christian Science Monitor, November 26, 2010
Nobody seems to have a sense of humor about John Cusack's "satanic death tweet" from Sunday night. Some news giants (Fox News anyone?) believe the actor is "calling for the death of Newt Gingrich and House Majority Leader Dick Army," according to
—Steve Spears, “John Cusack catches grief over 'satanic death tweet',” St. Petersburg Times, September 08, 2010
2010 (earliest)
Solly Forrell, the conservative blogger who called for Obama's assassination on twitter today, is being investigated by the Secret Service. Also, consider the phrase "death tweet" officially coined.
Chris King's death tweet:

Solly Forell's death tweets (which have since been deleted from Twitter):