Well to be honest they could do this with mobile phones without bluetooth and possibly more reliably: I had a conversation with a senior IT guy at a large UK retailer about using GSM picocells in the stores to do just this - the advantage here is the phone actively registers with the cell rather than being "discovered". I'm sure I've heard of some mobile telco that gives a discount if you are prepared to have location based advertising pushed at you via SMS. And then there is the RFID chip in your store card ...

/J\