There was a code to get it early but some people uploaded it.
Download it here to your computer
http://www.megaupload.com/?d=6GC02UBU
Unzip it with WinRar
On a CD or DVD make the following structure.
content - this is a folder
0000000000000000 - this is a folder (must have 16 zeros)
Place the unzipped folders and files into the 000... folder
Burn it to a cd or DVD.
The final structure on the disc should look like this
Content - Folder
0000000000000000 - Folder
5451884B - Folder
00080000 - Folder
9C02B874A2592D513BD4D7B9D0485CC26CE495E954 - File
Put it in your Xbox 360 and go to your games library and it will show up as a demo. Takes about 30 seconds to load and you just get a blank screen so wait it out.
Game seems pretty deep so far so not a lot of button mashing, but I just started playing.
BTW, I don't consider this piracy as it's a free demo anyway that's coming out later next week. You can just get it early this way.