Vice President of Autodesk Labs and inventor of DWF, Brian Mathews, reports: By default DWF files are made to be as secure as physical paper plots by only publishing the information you would get with paper. As such, the default settings prevent the publishing of layer information to the DWF file. If you want your DWF files to include layer information, you must enable this setting by configuring the DWF6 ePlot.PC3 file. To do so, go to File > Plotter Manager… and double-click on the DWF6 ePlot.PC3 entry. In the Device and Document Settings tab, click Custom Properties and then the Custom Properties… button.
In the subsequent dialog box, select Include Layer Information.
Note that the PUBLISH and Sheet Set Manager commands have their own settings which will override some of the settings you set in the PC3 file (like Layers)!