well according to the bloke who made this post you dont need any preorder or nothing.
I checked google, the demo is open for everybody in Europe, however if you are in the US, you need to have pre-ordered the game from Gamestop in order to receive the demo today. If not, you have to wait until the 26th for the demo.
It's a short demo, lasting about 10-15 minutes and is a small section of the first level. It doesn't reveal much about the story, it's just a demo to showcase the overall feel of the game. It took me a while to get used to because I was never into FPS games until I played COD4. It's different to COD4 and WAW because in those games, the person you control seems to move more freely and has no weight so the action is always fast. In this, the guy you play as feels heavy and as a result moves around slower. I'm not saying this is a bad thing, it's just different and more realistic.
The standard control setups are shit. A stupid feature is having to use R3 to aim with the sights. However, change the control setup to the one called 'Alternate 2' and the controls become (almost) identical to those of COD.
The cover system may take some getting used to but once you get the hang of it, it works really well. The only annoyance is that you have to hold L2 to stay behind cover instead of just pressing it once to snap to cover and then pressing it again to leave cover, but this is a minor issue.