Our brand-new Location Beacons w/ UWB can do that. These beacons have an ultra-wide band radio in addition to the Bluetooth radio, and with our new firmware, this means they can very precisely (up to a few centimeters) measure the distance between themselves.
We’re using this technology ourselves to automatically build floor plans:
… but you can do whatever else you want with it, we’ll have an API to access the distance estimations directly.
They are currently available for pre-order at http://estimote.com/uwb