|Jailbreaking an iOS device in order to install unofficial apps, add customization options or simply to unlock a locked device has gotten tougher, as evidenced by the hacking community's inability to produce an untethered jailbreak for iOS 6. In a recent tweet, hacker i0n1c has revealed that the forthcoming iOS 6.1 update adds "again tougher protections" to the codebase even compared to iOS 6, suggesting that security has been dramatically improved.
While many users have perfectly legitimate reasons (beyond just wanting to) for jailbreaking their iOS devices, because the technique relies on finding an exploitable "hole" in the OS code that could also be used for malicious purposes, Apple is naturally very eager to close up avenues by which unofficial or dangerous code could be injected into the device -- even though many "unofficial" apps are simply ones that were rejected by Apple for App Store guideline violations, mostly for altering core OS elements.
Closing down jailbreaking loopholes will also close off one of the principle sources of pirated apps, also giving Apple considerable incentive to cut off the practice. Holes in Android code are frequently used to install scamware, malware, privacy-compromising and even virus-ridden apps -- a growing problem for Google, though the ability to heavily customize and "root" Android devices is a major selling point to the most technically-proficient of Android's audience.
The hacker community believes that iOS 6 will eventually get an "untethered" (meaning "persistent through restarts") jailbreak, but that iOS 6.1 may represent the end of the free jailbreaking road. The security may simply have reached a point where only those likely to sell any remaining exploit secrets are likely to be able to come up with any.
Apple has made security a top priority on iOS, since it is the only platform where malware is all but completely unknown. Many of the security improvements made in iOS have also been transferred to the Mac as applicable, including complete sandboxing of applications and developer "signatures" on apps.
In his tweet, i0n1c refers to a "changing of the guard" that has brought much-improved security to iOS. It's unknown if this refers to Craig Federighi's recent promotion to handle both iOS and OS X, or if this is a reference to Kristin Paget, a top white-hat hacker herself who is now listed on LinkedIn as a "Core OS Security Researcher" at Apple.