After opening an iBeacon app we hard closed it: not just putting it into the background tray but swiping it closed entirely. The phone still detected beacons and sent a message through the lock screen, something which in the past was reserved for apps that were at minimum running in the background tray.
The functionality even works if you reboot your device: after you power down your phone and start it up again, it will continue listening for beacons even if you don’t open up the app again.
As Doug notes, this is an important change for how iBeacon works with iOS apps – it sounds like it's now more stable and it should always work, removing the need to make sure an app is running.
For retailers, museums, and any other place with iBeacon support, this means that launching an app the first time should be enough to have it always ready to listen for beacons in the future (unless the app gets uninstalled of course). And because the technology is based on Bluetooth LE, the impact on battery life should be minimal to non-existent. This seems like a great change for iBeacon.