Supposedly you'll get access to a demo if you've pre-ordered with Amazon. However there is no mention of it here and I've not received and email from Amazon despite pre-ordering months ago. Anyone know anymore?
If you have Borderlands on steam, go to your game library and right click Borderlands and there should be "View Game/CD Keys" or something like that. The Duke Nukem First Access code can be found there.
If you dont have Borderlands, other sites like TheHut.com are offering the First Access Demo.