The official Playstation blog today announced that from November the 18th the number of devices that can be activated on one PSN account for game content purchases will change. The number will be reduced to just two systems for both the PSP and PS3. This will not apply retroactively, meaning that previously authorised devices will still have access to any authorised content. But any game content purchases made after the 18th will be subject to this new policy.
So far this has only been announced for the AE, AU, GB, IE and NZ regions, with no word so far if this new policy will be introduced to the US.