[8ig8]

Apparently Apple is providing the NYC PD assistance in this regard:

http://www.nypost.com/p/news/local/nypd_apple_corps_PmTgzglh...

> Every time an Apple device is stolen, detectives attempt to get tracking numbers from the victim or online records.

> That number, known as the International Mobile Station Equipment Identity, is then shared with the officers in Police Headquarters who pass it on to Apple.

> The California-based company then informs the NYPD of the device’s current location — and it can track it even if it was reregistered with a different wireless provider.

[FireBeyond]

It's the IMEI, and Apple is certainly not alone in helping - any wireless provider can help, and indeed there are services available to law enforcement to determine if a cellular device is registered to another wireless provider, and contact them. Every cellular provider can also blacklist an IMEI, and it won't be able to be registered on any network, effectively bricking it.

[yardie]

* Every cellular provider can also blacklist an IMEI

This is very country specific and requires the cooperation of all service providers. Once the phone leaves the country and registers on different network the block becomes ineffective. Thieves here know the phone will be blocked so they send it on to Russia, for example.