Roach bait describes the most effective methods used to kill roaches, ones that function on viral principles. Roach Motels use it. Since roaches actually live in nests, it isn't enough to simply kill the one or two roaches that happen to wander into a trap, as plenty of others won't. Even if you kill all the adult roaches outside the nest, some live eggs remain in the nest and eventually hatch, which will leave you with the same problem you had before.
So, in order to kill the nest, Roach Motels don't trap individual roaches. Instead they lure roaches in, let them eat a pleasant tasting morsel of concentrated slow acting poison, and let them out. A slow acting contact poison with a lure in the middle will also do the job. Soon that roach dies but that poison remains in (or on) its body, which the other roaches soon eat, and become poisoned as well. In this way, each roach not only becomes a dead roach crawling, but also acts like a Typhoid Mary to the other roaches. Eventually the nest fills up with poisonous carcasses, turning the nest itself into a death trap.
Marketers also use "roach bait" to describe a particular method of guerilla advertising, showing all too clearly what they think of the "average consumer" (and how many companies view their potiential customers). It is generally used to target consumers who seem to have become resistant or inaccesible to other forms of advertising. Cigarette and alcohol companies have been known to use it.
Generally speaking, in this scheme, a marketing company pays a socially adept person, called a roacher, to use their product visibly and convincingly: to use the product in locations where they will be seen doing so by the target consumers and to talk up their product to people they befriend while doing this. If it works, the roacher will be able to sell consumers on their product without those consumers even noticing it. Then, like roach bait, those consumers will try that product themselves and/or tell their friends about, who then try and/or tell, etc. - crafting a planned marketing campaign that looks like spontaneous word of mouth.
This sounds great, but a roach bait campaign must meet certain strict requirements, otherwise it will fail and often backfire. Overall, roachers must appear to be a peer of the roachee(s) with no vested interest in selling a product. Thus, since everyone knows celebrities get paid to endorse products, celebrities can't roach bait (except possibly to other celebrities). More importantly, a roacher must never appear to be selling the product - the most they can do is offer free samples out of enthusiasm and personal kindness. If at any time a roachee finds out they are being or have been marketed into liking the product, they tend turn on the roacher (and that product) in anger over being mislead. For a perfect example, look at most people's reaction to Amway when they find out their "new friend" offering them a "great opportunity" is a actually a distributor trying to sell them a franchise: "You tricked me!"
More recently, perceiving roach baiting's image problems, some marketers have switched to "brand baiting" instead. Still, it seems highly likely roach bait campaigns continue to occur - just very quietly.
Be carefully who you listen to. You are what your friends eat.
Thanks to No Springs for mentioning celebrity endorsement and helping me clarify.
Also submitted to wikipedia.org.