If you take a peek at the various .int files in the System directory of the demo, you'll at some point come across what appears to be a list of the levels.
SPLevel0=Duke Lives
SPLevel1=Damn! It's Late...
SPLevel2=The Duke Cave
SPLevel3=Mothership Battle
SPLevel4=The Lady Killer
SPLevel5=Vegas in Ruin
SPLevel6=The Duke Dome
SPLevel7=The Hive
SPLevel8=Queen Bitch
SPLevel9=Strip Club
SPLevel10=Crash Course
SPLevel11=The Duke Burger
SPLevel12=The Mighty Foot
SPLevel13=Ghost Town
SPLevel14=Highway Battle
SPLevel15=Dam Top
SPLevel16=The Shrunk Machine
SPLevel17=The Forkstop
SPLevel18=Generator Room
SPLevel19=Underground
SPLevel20=The Clarifier
SPLevel21=Blowin' The Dam
SPLevel22=Final Battle
SPLevel23=Press Conference
These (and from what we've seen of the game) match up very well with the following images, which were leaked around May 09, except the game continues after blowing the dam and defeating the boss (and starts to get good IMO).








Perhaps they (Gearbox) thought it would take too long to fully complete, so decided to cut it out. If so, I can only hope it gets completed and released as DLC (creepy dream sequence, playing as bombshell and fighting Duke clones sounds awesome).