This is due to happen during a November 15th maintenance by Sony on their PlayStation Network.
So get your games bought before the change.
That's not all, the same will apply to PS Vita downloads
Sony have removed the backward compatibility in the PS3 removed other OS.
From the official Sony PlayStation blog...
For game content purchased after November 18, 2011, a new policy will apply and the number of devices that can be activated will be as follows:
- PS3: Users will be able to play the game on up to 2 activated PS3 systems.
- PSP: Users will be able to play the game on up to 2 activated PSP systems.