Dead Man Walking

As an idiom, the phrase "dead man walking" is most infamous as a call-out once traditional in American prisons; when the wardens would lead a man on Death Row down the hall, declaring "Dead man walking! Dead man walking here!" Dead Man Walking means Your Days Are Numbered and you and/or the people around you know it. Now that prison wardens no longer use it (least not where they can be noticed), modern use of the term broadened somewhat to other types of doom, even benign "doom" such as losing a job (which may actually be the original meaning, but so far no one's been able to trace it conclusively).

Does not have anything to do with The Undead but is often used that way Just for Pun. Compare Your Days Are Numbered, Who Dunnit to Me?, Incurable Cough of Death, Doomy Dooms of Doom.

Invoked by Dead Star Walking. Sometimes used to describe a state in a game that's Unwinnable by Design.

Examples of Dead Man Walking include:

Film

As a dog is being led away to be put down at the pound that Lucky's taken to in the Eddie Murphy film version of Doctor Dolittle, one of the dogs comments "dead dog walking."

Literature

Stephen King's The Green Mile has the warden Percy Wetmore, who tries to carry out the traditional callout as he leads John Coffey to his cell in E block. The protagonist and supervisor tells him to shut up - by the time the story is set the phrase is becoming deprecated so it established Percy's character as a Jerkass and a later villain.
The title of the book and subsequent film and stage adaptation Dead Man Walking, Based on a True Story of a nun who became the spiritual advisor to a Death Row convict.
In Rudyard Kipling's The Jungle Book a wolf who loses a challenge is called the Dead Wolf as long as he remains alive, "which is not long as a rule".
Literally the title of a Warhammer 40,000 novel, showcasing the incredibly dour Death Korps of Krieg as they battle the implacable Necrons. The title comes from the fact that the Kriegers have no moral compunction against sacrificing their own men to kill the enemy, and are all drilled to have no dreams, aspirations, or visions of the future beyond service in the Imperial Guard—and their inevitable death. They are so cold and emotionless as to creep out their fellow Guardsmen.
- It helps that all Kriegers are cloned.
Another WH40K example: Within the Night Lords legion there was a practice of placing a suspended execution upon legionaries who warranted death, but were too valuable to be executed at present. Their gauntlets were painted arterial, or "sinner's" red and their lives continued at the whim of their primarch.
Variation from The Fifth Elephant:

Vimes realized he was a dead man bathing.

One of Jasper Fforde's Thursday Next books features a pair of red-shirts called Dedman and Walken, who are (obviously) doomed.

Live-Action TV

On MythBusters, the Narrator often intones the phrase jokingly when referring to the (inanimate) victim of the day's experiment - usually Buster the crash test dummy but there's been "Dead Car Driving!" and others.
An NCIS episode titled "Dead Man Walking" involves a victim who has gotten radiation poisoning and the team has to figure out whodunit before he dies.
- NCIS also brings it up in the episode "Forced Entry":

Marine Sgt. Hegarty: You never mess with a Marine's coffee, if you want to live, Agent DiNozzo.
Tony DiNozzo: [laughs] That's right. Dead man walking!

The Thick of It episode "The Rise of the Nutters"

Malcolm Tucker: [Ben enters a party] Oh, here he is. Dead man walking.

In the third episode of Stargate Atlantis Rodney who was starving, because an Ancient device didn't allow him to feed, abused this phrase and its variants.
An episode of Torchwood is titled "Dead Man Walking". In which Jack tries to resurect someone with a magic resurrection gauntlet against the warning of his companions.
In the Episode "Bridesmaids Revisited" from Gilmore Girls Rory and Logan are at the wedding of Logan's sister. They encounter the groom in the hallway, and Logan has to help him get ready for the ceremony. He grabs his shoulder and calls out that phrase as they walk to his room.

New Media

There's a website with a feature called Dead Bro Walking, that focuses on the Black Dude Dies First trope.

Professional Wrestling

The Undertaker, in 2002 started using theme music with ominous bells and the phrase "Dead Man Walking". Fans remember it as the year he beat Hulk Hogan.

Video Games

In Max Payne part two there is a minigame called Dead Man Walking in which you fight infinitely respawning enemies, trying to take as many as you can with you.
Dragon Age. All Grey Wardens, due to the Taint. Also, Wynne and, more figuratively speaking, the Legion Of The Dead. Their initiation is a dwarven funeral rite. Since they are already dead, they have nothing to fear.
In Crysis, everyone who puts on a Nanosuit. And more literally, Alcatraz, the Player Character in the second game.
Thane Krios from Mass Effect 2. He has an incurable disease that slowly destroys his ability to breathe, and all the doctors say he'll be dead in less than a year
- The doctors say he'll be dead in around three months. He last talked to them nine months ago. And if he survives even the Suicide Mission, he manages to drive off Kai Leng despite his disease leaving him barely able to breathe.

Web Comics

In an issue of Tales Of Zenith a gallon of milk in a store wants to sneak to the front of the display case. The reason is, his expiration date is yesterday, which means he'll be burst and dumped down the sewer if he can't escape by being bought. The cottage cheese and sour cream notice this, and they call out Dead Milk Walking!

Western Animation

The second Futurama Christmas episode has "Deactivated robot walking" when Bender's being taken to the ~~electric~~ electromagnetic chair.