@hitman; It was included as bonus content if you purchased God of War Ascension. It's not that you had to BUY the demo, but you weren't getting the demo unless you owned/borrowed/rented GoWA, because it was unlocked from the disc.
So if you have a friend with the game borrow the disc and unlock the demo on your console. It downloads from the PS Store and then its on your console permanently. The demo is REALLY short in comparison to what they let us play at PAX East, but its still fun to get the mechanics down of the game and work on your strategy for taking down infecteds.