Update: 10/08: xMatters for iOS 2.7.1 has provided a new workaround to this issue. Just launch the xMatters app once after upgrading to iOS 8 and you will automatically be re-registered for push notifications
iOS8 just hit the App Store and with it comes thousands of xMatters users upgrading to an entirely new smartphone operating system. A pretty amazing feat that Apple has accomplished, but unfortunately they haven't fully worked out all of the issues that go along with making such a major change to an important personal device.
In this case:
The iOS8 upgrade will require a user to manually launch xMatters for iOS v2.7.1 once to re-register their device with xMatters. Until this action is performed the user will not receive push notifications.
xMatters for iOS v2.7.1 is available now from the app store.
See this article for more details