While perusing the code for iOS 5 beta 2, the members of the iPhone Dev Team discovered Apple is putting measures into place to make it more difficult for iOS 5 users to rollback to earlier versions of iOS.
It isn't uncommon for more advanced users to want to try out the latest non-jailbroken software and then revert back to an earlier version so that they can take advantage of their jailbroken features. The current workaround is for users to backup their SHSH blobs before upgrading the software. This ensures that an older version of the OS and firmware can be restored at a later time.
With iOS 5, Apple is changing the way that the blobs are created. The blobs will be regenerated every time a device is rebooted, meaning that simply having an old version of the key around won't allow users to install an older version of the software.
While this doesn't prevent groups like the iPhone Dev Team from finding exploits and providing tools so that users can jailbreak their devices, it might make keeping a device in a jailbroken state more difficult.
With iOS 5, Apple will also be looking at doing over-the-air OS upgrades, meaning that the software can update itself without needing to connect with iTunes. In theory, this means that Apple could push out small updates to patch exploits, preventing a user who has agreed to install said update from jailbreaking their device.