Well, some places call it a demo, some call it a beta demo. Nonetheless, the sites where you can download it at are :
3D Downloads, 4players.de, ActionTrip, AusGamers, ComputerGames.ro, eXp.de, FanGaming, Fileaholic, FileFront, FilePlanet (registration required), FileShack, Gameguru Mania, Gamer's Hell, and PixelRage.
(Thanks goes to Blue's News for that list)
I suggest GamersHell personally but the servers were packed this afternoon. Best of luck for those who have to sit in lines.
Also, it seems that there is an unofficial torrent floating around that is just the demo.exe. That is also a good way to go as the number of seeds were pretty high. If only the companies would start using torrents to distribute their big files everything would be better.