iOS 8 randomises the MAC address while scanning for WiFi networks. Hoping that this becomes an industry standard.
— Frederic Jacobs (@FredericJacobs) June 8, 2014
By randomizing your phone's MAC address (its individual thumbprint—nothing to do with Macintosh), iOS 8 will make it much more difficult for wifi networks to sniff out your device and track where you've been.
A small industry has mushroomed up around these new tracking abilities, documenting where you go—and possibly keeping that data indefinitely—without you knowing it. Corporations use this data to find out who's been through their stores, how long they linger in any given section, and how to target customers accordingly.
Policing organizations have also been dabbling with digital tracking using devices like the Stingray, which harvests MAC addresses and other data without a warrant. The Electronic Frontier Foundation rightly calls the Stingray an "an unconstitutional, all-you-can-eat data buffet."
And in Seattle, we saw the quiet installation of a wireless mesh network throughout downtown Seattle that also had the capacity to track MAC addresses. The Seattle Police Department, which bought the mesh network with Homeland Security grant money, never confirmed that it planned to use the technology in that way—but it also declined to answer detailed questions about how, exactly, it would use those tracking capabilities and who would have access to the data it could collect. (Shortly after a story about the network was published in The Stranger, the SPD announced it was turning off the wireless mesh network indefinitely.)
Tech sprints while the law crawls, so we don't know whether the courts consider the blanket surveillance of where your phone goes a violation of Fourth Amendment protections against unreasonable search and seizure, or of First Amendment protections of political speech.
But we do know that tracking everyone who was at a particular protest, for example, and then surveilling and disrupting their lives would be a dream come true for some detectives—police in Miami-Dade County, for example, once put in a request to acquire a Stingray specifically to monitor protests at a World Trade Conference.
In the surveillance race, iOS 8 is a heartening leap forward.
As Phil M points out in the comments—and I should've mentioned in the post—when activist Aaron Swartz did a similar thing with his MAC address, the government used that as evidence of criminality in their indictment. (Swartz later committed suicide.)
So while this is a leap forward, and while it should be the direction in which the industry bends, it's also a sad example of a) us having to buy Constitutional protections from corporations, and b) that, in some matters, when an activist does something it's considered evidence of a crime, but when a business does the same thing it's considered a feature.