dng is not a proprietary format. Adobe produced the specification for dng (Digital Negative) and released it to the public. They supply it free together with free editors and the source code is available to all. It's much like Linux, someone has to start it, in this case it was adobe.
You can download the specification, which describes a non proprietary file format for storing camera raw files that can be used by a wide range of hardware and software vendors for free. See this adobe page for info.
The Digital Negative (DNG) format is a non-proprietary, publicly documented, and widely supported format for storing raw camera data. It is hoped that camera manufacturers will adopt it instead of producing all the proprietary RAW formats that they do.
Sony RAW is proprietary and Raw files from an A100 are not compatible with A700, A900, a77 or A99 RAW files, you have to get revisions of Sony's or Adobe's software to read the latest Sony camera RAW.
I personally do not use dng as there seems no real benefit, at this time, to doing so. However if all the camera manufacturers were to adopt it instead of making their own proprietary RAWs, then I would.