Tl;dr: Not that we know of. At least not deployed anywhere outside a lab. Some proposals have been floated, however.
This post was inspired by the recent report by the US Congress Government Accountability Office on ADS-B, which was discussed in many articles around the web, such as this one, for example:
Let’s have a look at the source, these are quotes from the GOA’s actual document, citing some of the work from contributors here at OpenSky: