Mac – Monitoring the Mac App Store for free applications

From Daring Fireball:

Panic on the Mac App Store

Cabel Sasser on what it’s like for existing Mac apps that are now in the App Store:

PS: If you’re a current customer, there’s no way to convert a previous purchase into a Mac App Store purchase — that requires a re-buy. But remember, what you have now will continue to work just fine.

UPDATE: The Mac App Store may show software bought from us previously as “Installed”, even though they’re two different licenses. You will not get Mac App Store auto-updates unless you purchase from the Mac App Store. To re-enable the “Purchase” button in the Mac App Store, just drag the app to the trash. Your preferences/sites will not be affected.

In other words, for apps that use the same bundle ID for the both the Mac App Store and non-App Store versions, the Mac App Store will not install updates to already-installed non-App Store versions, but it will recognize them as being already installed. If you want to get updates via the Mac App Store, you’ll have to re-buy the app through the App Store.

As Joe indicated, the reason why you have not found an app to do this is because apps simply can't.

Backgrounding apps can do very little, and even things that they can do are limited to generating a notification which you must acknowledge in order for the app to actually carry out it's task. For example; Skype can background and detect a call, but you must still acknowledge the notification to open the app, and have anything meaningful actually occur. Additionally, backgrounded radio stations / audio streams can continue to stream, and even change channels or support some feature that responds to the "skip forward" and "skip backward" media controls, but if the network connection is broken the app is suspended and will not do anything further until you re-open it.

Sadly, this is also not part of the accessibility features built into iOS, perhaps you could file a bug and request the feature be added in a future software version?