Undercover cops in New York are riding the subways with iPods on to entice robbery. Is that a form of entrapment? If not, why not?

Enticement is not Entrapment.

Entrapment is fairly specifically defined in law… It happens when the police create a situation where someone commits a crime that they would not otherwise be inclined to commit.

From the circuit court ruling:

Why is sin so sweet?

Government agents may not originate a criminal design, implant in an innocent person's mind the disposition to commit a criminal act, and then induce commission of the crime so that the Government may prosecute. Sorrells v. United States, 287 U. S. 435, 442. Jacobson was not simply offered the opportunity to order pornography, after which he promptly availed himself of that opportunity. He was the target of 26 months of repeated Government mailings and communications…

Not at all.

The most famous example of Entrapment is probably Jacobson v. United States, a case from the early 1990’s about child porn. In this case, the government repeatedly offered Jacobson illegal content, and provided a whole bunch of reasons why he should purchase it under the guise of free speech, etc.. When he eventually did (after two and a half years of basically being pestered about it), they arrested him.

Have you ever seen a woman having sex with a dog?