According to a post on the US Playstation Blog, Cristian Cordona, an Associate Product Manager, Software Marketing states that for those that pre-order Resistance: Retribution you will get a code which will allow you to download the demo two months before the game is released.

Customers who pre-order Resistance: Retribution from Amazon, Best Buy, GameStop, or Game Crazy will receive a voucher code granting them access to the Resistance: Retribution demo almost two months early! The demo features single player gameplay and the option to try Resistance Connect with both the Infected mode and Resistance: Retribution Plus.

How to Activate Resistance Connect!

1. Insert and load Resistance: Retribution on the PSP.
2. Insert and load Resistance 2 on the PS3.
3. Connect PSP to PS3 via USB Cable.
4. From “Start” menu on the Resistance 2 menu, select “Options”.
5. Scroll to “Connect PSP”.

From there, you can activate Infected Mode and Resistance: Retribution Plus!

You can buy the game through Gamestop here.

They also released some screens from the demo which you can see here.