I share this in the hope that it will help out someone else in the future.

Today I was surprised to see that when I tried to run an app on one of my devices I got a message saying that the app had been revoked and I must uninstall it.
This surprised me as it was an app that is still in development.
After a bit of poking around I discovered that the developer registration for the device had expired. Yes, when you "unlock" a phone for development it only stays in that state for a year.

Dealing with this was simply a case of removing the device from the "my registered devices" list in app hub and then unlocking it again. I initially tried to unlock it again without first removing it from the list but this failed to work, despite the tool saying that unlock had been successful.

As we approach a time when devices became available to all there are likely to be lots of developers who start to see the same situation as they too will have had [unlocked] devices for a year.

Please also note that when the unlock has expired it also not possible to deploy to a device from within Visual Studio (or Blend) and you'll see a message about checking the device is developer unlocked.

Source:  http://blog.mrlacey.co.uk/2011/09/renewing-developer-phone-registration.html