Max Moser just published a very interesting (and frightening) paper about Bluetooth sniffing :
“During the last year, rumours had come to my attention that apparently it is possible to transform a standard 30USD Bluetooth(r) dongle into a full-blown Bluetooth(r) sniffer. Thinking you absolutely need Hardware to be able to hop 79 channels 1600 times a second I was rather suspicious about these claims. This paper is the result of my research in this area, answering the question whether it is possible or not…”