As published on July 17 in Autodesk DWF Viewer 7.0 Known Issues, we are wary that not everyone reads the read me; however, many people do have blog updates delivered via email, RSS feeds, etc. With this in mind, the following has been culled from the AutoCAD 2007 read me for DWF files:
Raster images in DWF underlays are not plotted when plotting to DWF.
Plotting DWF underlay files that contain raster images can produce unexpected results.
So when publishing or plotting a DWF file from a DWG that has another DWF as an underlay, please keep this in mind. These issues are on the list for consideration for a future release of AutoCAD.