Sony has announced a change to its usage policy, which will reduce the number of systems on which a downloaded game can be played.
All PSN games purchased after November 18 will only be playable on a maximum of two systems. The rule applies to PS3 and PSP games alike, but does not apply to games bought before the new rule comes into effect.

http://www.next-gen.biz/news/sony-tw...ivation-policy