Owen Wengerd has an interesting article on his Outside the Box blog:
Owen concludes with:
If you are providing electronic files, don't rely on copyright law alone to protect your intellectual property. This case reinforces the 3 C's for protecting AutoCAD DWG files: copyright, contract, and CADLock.
Even with the existence of the DWF Underlay command, sharing low precision DWF files (you can set the vector DPI to 400) instead of the original DWG is an alternative as well. A DWF file is not your source intellectual property. It takes effort for someone to convert it back to a DWG. In this case, the customer who requested the DWG files for "personal use" could probably have used DWF files just as easily,