I don't know BT. I am with VM. My only thought is that somewhere in your BT wireless device, if it has a GUI (user interface) there may be a link to a log file. This may tell you what devices have been connected. However, as KB said the tunneling to BT may prevent hotspot users being logged.
You sound like you regret having BTFON, but don't forget you should be able to log on to the millions of FON spots worldwide not to mention BTFON spots at home. It depends how much you use your phone for shifting data I suppose. This can be very expensive via 3G.
Virgin Media: 100mb/10Mb , land line, mobiles x2, TV - VIP HD box + TIVO 1Tb box.