All apps in the Windows Phone Store show they will work with Windows Phone 8.1
We took a look at several apps in the Store, and they are all listed the same. The change apparently happened en masse overnight. That development bodes well for those that are waiting impatiently for the official announcement and demonstration of Windows Phone 8.1 because it means there should not be any compatibility issues for apps running on the updated OS.
We are just a little over two weeks away from the Microsoft //Build/ developer conference where it is expected that all will be revealed.
sources: WindowsPhoneApps.es via WMPoweruser
Things that are NOT allowed: