On October 15, Take 2 will be bringing out a Game of the Year-edition of Borderlands which will include all the expansions and patches that have been released for that game so far (which is quite a lot, actually). If you buy that, you will get a key to the "Duke Nukem First Access Club", from which you will - at some point in the undisclosed future - be able to download the demo for Duke Nukem Forever.
You'll also get a "wealth of goodies", but no one really knows what that means yet.