It's no secret that Bluetooth devices that are in discoverablemode are... well, discoverable, so we figure it was just a matter of time before an independent organization set up a massive network of transceivers to track their every move.
The thing is that this can only be done when your Bluetooth is on AND your device is set to discoverablemode, though there are two Nokia handsets (the 6310 and 8910) which are vulnerable even when they are not in discoverablemode, and if you have one of these you need to get a firmware upgrade to prevent the attack.