The second iOS 5 beta was seeded to developers last week and it looks like Apple has "stepped up their game" to combat jailbreaking, as reported by Gizmodo. The latest beta also takes measures against downgrading.
According to a blog posted by iPhone Dev-Team (the hacker group responsible for PwnageTool and Redsn0w jailbreak tools), Apple has altered the iOS boot sequence to require a signed authentication ticket- which is yet to be (and may never be) hacked.
This renders current "untethered" jailbreaks, used in tools such as PwnageTool and Redsn0w, effectively useless for iOS 5 at the moment.
There is another tool that uses an exploit earlier in the boot process called limera1n that will still function for the foreseeable future, but is a "tethered" jailbreak.
While I have the utmost confidence in the Dev-Team hackers it appears Apple is really ramping up measures against jailbreaking in iOS 5. Some find it disconcerting that iPad 2 has still not been hacked either. The cat and mouse game has been raging since 2007 but it looks like the real battle has just begun.