The demo will be available to download via the PSN after the PS Store updates tomorrow. In the mean time Sony have had this to say about the demo:
Become Kratos and escape the epic Prison of the Damned. God of War: Ascension‘s demo will allow players to explore and master the legendary Spartan’s new advanced combat:
- Make your stand against a Titanic beast and its infected legions.
- Unleash your Rage in fiery explosions that burn your enemies to ash.
- Impale enemies on your chained blades and use them in combat as living wrecking balls.
- Wield the weapons of your enemies in countless new combos.
God of War Ascension will be available on PS3 from March 12, 2013 in the US and March 15 in the UK.
Source: EU PS Blog